Powering your house from the sun.<\/p>\n
In 2016 we sold a lot of mature timber and came into a lot of money in one year. We needed a
\ntax write off and so looked into solar for our house. Presently we are running the house and two
\nelectric cars and getting a check every year in February for excess production. When we began
\nthis journey 4 years ago, I didn\u2019t know what I know now. We made a few missteps along the
\nway. It is my hope that you will find this informative and not make the same mistakes we did.
\nBy now, you have seen \u201ctoo good to be true\u201d and \u201cno money down\u201d solar systems for sale on
\nthe internet. Keep in mind that nothing is free and if you wouldn\u2019t buy a car from a buy here pay
\nhere place, you shouldn\u2019t buy \u201cno money down\u201d solar. A chapter will be devoted to this subject.
\nSolar has become quite affordable lately and many people don\u2019t even know where to begin. I
\nhope to answer a lot of questions and prime you to ask the right questions when it is time for
\nyou to pull the trigger.
\nFor the sake of this article, when I refer to a solar system, I\u2019m referring to a solar PV
\n(photovoltaic) system. There are thermal solar panels that heat water, but my primary interest is
\nin electrical panels.
\nSolar PV systems fall into two categories…. Grid Tie and Off Grid.
\nGrid Tie systems are the cheapest and most commonly sold. With grid tie system, you treat the
\ngrid like a battery or a bank account. During the day your electric meter will run backwards
\npushing power into the grid just like you put money into a savings account for a rainy day. At
\nnight, your meter will once again run forwards eating away at that excess power you have in
\nsavings. Your meter will run backwards more in the change over seasons like fall and spring,
\nand you will tend to draw more power from the grid in the heat of summer or cold of winter.
\nFirst and foremost, a Grid Tie system does not make power during a power outage. As a matter
\nof safety, a Grid Tie solar system will not make power until the grid has been up for 5 minutes.
\nThis is to prevent back feeding power and potentially killing a lineman.
\nOff-Grid systems have the ability to continue operation when the grid is down or when you
\nchoose to not have electric service. Ideally, they will use as much of your home grown solar
\npower as possible before drawing from the grid. If you so choose, an off-grid capable system
\ncan also back feed the grid just like the grid tie system and earn you credits with your local
\nelectric company.
\nOff Grid systems by definition must use batteries. Batteries until lately have tended to be lead
\nacid but Lithium batteries are very affordable now. We will discuss batteries more in depth later
\nin this article.
\nSo let\u2019s get cracking!<\/p>\n
The first thing you need to know is how much power you will need to make. Since electricity is
\nproduced and sold by kWh, this is what we need to determine. Look on your electric bill and
\nfind the kWh used for the month. Totalize this for one year and divide by 365. For example, you
\nfind that your total electric usage for the year is 18,250kWh. 18250\/365 = 50kw per day. So
\nyour goal is to make 50kw a day on average for a year.
\nAllow me to pause here for a moment. It is very important to know how much power you are
\nusing. Do not guess. It is equally important for you to optimize your house before you do your
\nsolar calculations. The easiest gain can be had by switching your lights to LED. LED lights are
\n10x more efficient than incandescent, and 2.5x more efficient than fluorescent lamps. In my
\ninstallation in 2016, we failed to take into account the lighting in the house and a few 4 tube
\nfluorescent fixtures in the basement that stayed on 24\/7. Installing three light switches in the
\nbasement and swapping all the lamps in the house took our energy usage from 50kw per day to
\n34kw per day. Because we found this out after installation, we overbought solar.
\nSmart money buys a kWh meter for your house that shows usage minute by minute. There are
\nmany to be had on amazon in the $150-$200 range. Spending a little on a power meter now
\ncan save you from spending more on solar than you need. A personal favorite is a power meter
\ncalled egauge (www.egauge.com). This unit has 16 inputs for current measurement and can
\nkeep one minute recordings of power usage for 30 years. It can also interface with many solar
\ninverters going forward. For example, it is networked to my inverters and can display battery
\nvoltage and state of charge.
\nHere is a sample display from the egauge system. It shows solar production in green and
\nusage in red. We have programmed the system to be able to track production from two different
\nsolar inverters and also to show us heat pump usage and car charging usage. Below we see a
\ncloudless morning becoming cloudy in the afternoon. The short red spikes are the HVAC
\nsystem cycling on and off. The larger red places are two electric cars charging. If you look
\nclosely at the green portion, you\u2019ll see two dashed lines. The higher one is the SolarEdge 12kw
\ninverter that is on the SW panels and the smaller production is from the SMA Sunny Boy that is
\non the SE panels.<\/p>\n
We used the egauge to identify a base load in the house. In our case it was three lights in the
\nbasement that were on all the time. We\u2019ve also used to egauge to realize that the horse trough
\nwas left running or the toilet flapper was stuck causing the well pump to rapid cycle. We have
\nconnected current measurement transformers to the feeds for our electric cars and our heat
\npump so we know in an instant how much we spent on car charging last year. Having an
\ninternet connected power meter continues to save us money.
\nFurther, once a solar system is installed you can use a power meter such as egauge to verify it
\nis working. I know of many cases where a family waits for their first electric bill to see how much
\npower they made only to find out that the installer didn\u2019t turn on all the inverters or there is a
\nwiring error.
\nSo now that you\u2019ve optimized your energy usage, let\u2019s get back to determining how large of a
\nsolar array you need.
\nThe next question to answer is how many hours of sunlight does your area get each day on
\naverage? You can check an insolation chart or use this one
\nhttps:\/\/www.altestore.com\/howto\/solar-insolation-data-usa-cities-a35\/ . In our example system, I
\nfind that the nearest city to me is Charleston SC and the average full sun hours are 5.06.<\/p>\n
So let\u2019s take the 50kw per day and divide by 5.06 hours of sunlight…. The answer is
\napproximately 10kw. Congratulations, you now know you need a 10kw solar system.
\nA typical turn key installed grid tie system cost $3\/watt. Recently Tesla has offered turnkey
\nsystems for $1.50\/watt. So our example system will cost between $15,000 and $30,000.<\/p>\n
As we begin our journey, we start with the panels themselves. Panels are typically 39\u201d wide and
\n65\u201d long or even longer. An individual panel typically produces 250 \u2013 400watts. If you need a
\n10kw (10,000watt) system, you\u2019ll need between 25 and 40 panels. Each panel has an
\naluminum frame and a clear lexan covering. The panels should survive hailstones and snow
\nloads for 20 years. They can be mounted in portrait (vertical) or landscape (horizontal) format.
\nThe panels will have a pair of 3 ft wires attached to the back. The idea is that the wires from
\neach panel can reach its neighbor\u2019s wires. The connector used is MC4 and is completely
\nwaterproof. These connectors should never be plugged or unplugged while the system is
\nmaking power. They will arc and the arcing will degrade the connectors. Break the connection
\nat the solar inverter first, then plug and unplug the MC4 connectors.<\/p>\n
Please note that most panels lose ~0.6 to 1% of capacity per year of use. For ease of
\nconversation we\u2019ll call it 1% a year and accept that at the end of 20 years the system won\u2019t be
\nbroken, it will just be producing 80% of its original power. The smart person includes this in their
\nproduction calculations.<\/p>\n
Panels can be mounted either on your roof or on a ground mount. With a roof mount you must
\nuse the pitch of your existing roof even if it is not optimal. For example at the latitude of
\nColumbia South Carolina, the optimal roof pitch is around 8 pitch (8\u201d rise every 12\u201d run). In
\nupstate New York the optimal angle may be 12 pitch (12\u201d rise every 12\u201d run). In most cases
\nyou\u2019ll probably find that your roof pitch is too shallow. You can make up the lowered efficiency
\nby simply adding a few more panels.
\nHere is an example of one type of roof racking system. Each mounting pedestal is screwed to
\nthe roof with a lag screw. A stamped piece of flashing is used to divert water around the
\npenetration and the hole is typically filled with caulk before the screw is installed. The panels
\nare then clamped on their edges to the pedestals and to each other.<\/p>\n
Building codes will require panels on the roof of a habitable structure to have a 3ft easement
\naround the sides and top. This is so the fire department can get on the roof safely should there
\nbe a fire and the roof needs ventilation. This 3ft easement also applies to roof valleys but in this
\ncase it is 18\u201d on either side for a total of 3\u2019.
\nVery often installers must skip a space due to a plumbing vent. Be sure to check with your local
\nplumber to see if you can replace the vent pipe with an air admittance valve. These are typically
\n$20 at Lowes or Home Depot and allow proper plumbing venting from inside your attic.
\nYou can also ground mount solar panels. The advantages are that the panels run a little cooler
\nthan on a roof and you can optimize the tilt for your area. The disadvantage is they take up
\nspace in your yard, if you have livestock, cows might push them over when rubbing and
\nsomething from your lawn mower might hit them.<\/p>\n
The solar panels are daisy chained together using waterproof connectors that are attached to
\nthe back of the panels. Just as a bunch of D cell batteries are in series in an old school boom
\nbox, so do solar panels connect in series. Each panel can be thought of as a battery that
\nmakes 40v. By comparison your car battery is 12v and a D cell is 1.5v. Typically 12-14 panels
\nare connected in series to make ~560v DC. One series of these panels is called a string. Each
\npanel makes about 7-9amps. By comparison a coffee maker uses 10-12amps. Because the
\ncurrent (amps) is so low, the wires from the roof to your inverters are standard 14AWG
\nhousehold wires. In our 10kw example system we determine that we need 3 sets of 14 panels.
\nSo we will run 3 pairs of 14AWG wire from the roof to the inverter on the side of your house. In
\nthis example, we have 3 strings of fourteen 250w panels for a total of 42 250w panels. The
\nmaximum power this system can produce is 10,500watts.<\/p>\n
The astute reader will realize that having 3 pairs of wires with nearly 600v on them could be
\ndangerous to handle and they would be correct. The national electric code recognized this
\nproblem as well and requ
\nires all panels to be disconnected and the system voltage to shut off within 30 seconds of an
\nemergency. Prior to 2019, the code only required the wires coming from the roof to be
\ndisconnected, but NEC 2017 Rapid Shutdown requires module level disconnection. This is
\naccomplished by attaching switch modules on the back of each panel or sometimes one per pair
\nof panels. The inverter system will communicate with these modules either by radio or by
\ncommunication signals overlaid on the power wires. Depending on which inverter brand you
\nuse these modules may be called optimizers or they may be referred to as a Tiga system.<\/p>\n
The next topic for discussion is that of shade. Back to our old school boom box with 8 D cell
\nbatteries. If one battery goes dead, the boom box won\u2019t work. With solar panels, shade cast on<\/p>\n
one panel in a string will kill the entire string in the same way. Solar inverters such as Morning
\nStar, SMA, Schneider electric have no solution for this problem. The only solution is to try to
\nwire the panels at risk for shade into their own string. SolarEdge uses optimizers to intelligently
\nfigure out which panel(s) is\/are shaded and electrically skip around them. These optimizers
\nmount on the back of the panels and communicate with the inverter about once a minute. If a
\npanel is found to be holding back the rest, it is bypassed.
\nWhen considering a site for solar you should look for trees, chimneys and TV antennas.
\nAnything that casts a shadow wider than about 4\u201d could hinder a significant amount of power
\nproduction. There is an app for iOS called sun seeker that overlays the path of the sun in the
\nvarious seasons of the year onto the camera view. Use this app to locate trees that may cause
\nshade.<\/p>\n
As we all know, the sun is higher overhead in the summer and closer to the horizon in winter.
\nThe optimal angle for a solar array facing directly south is that of your latitude. For example,
\nColumbia South Carolina is at 34 degrees north, so our panels need to be 34 degrees off
\nhorizontal. Locate your latitude on the chart below to see if your roof is at the optimal angle. In
\nour example, 34 degrees is an 8 pitch roof. Since you cannot control the pitch of your roof, the
\nonly solution is to add more panels to compensate for the reduced production. In our case we
\nhave a 16.5kw system that makes a maximum of 14kw due to our roof pitch being too shallow.<\/p>\n
Azimuth is the angle of your solar array as compared to south. If your array faces due south, it
\nhas an azimuth of 0 degrees. If your array faces south west it has an azimuth of 45 degrees.
\nNaturally there is a sweet spot for the tilt of an array if it isn\u2019t facing due south. A competent
\nsolar installation company can determine this angle for you or you can use an online calculator
\nto find the correct angle for a given location and azimuth.<\/p>\n
For most applications you are confined to the existing azimuth of your roof and the existing
\npitch. For ground mount applications, you can control the azimuth and tilt of the panels.
\nUsing the calculator available at https:\/\/www.sunnydesignweb.com\/sdweb\/#\/Home , we can
\ndetermine that our example 10kw system with 42 panels can make us 15,300 kwh per year. At
\n12c\/kwh, that is $1836 per year. If the combination of roof pitch and house direction isn\u2019t
\noptimal, it could cost us $150 in electric production per year, or put another way, we would have
\nto add three additional panels to the roof to make up the difference. In my example, I took the
\nworst case by comparing a panel mounted facing south at the optimal angle to a panel mounted
\non a 12 pitch roof facing south west.
\nWhy wouldn\u2019t you want an array to face directly south if it was possible? In off grid applications
\nyou want to make more power in the morning to recover your batteries as quickly as possible in
\nthe morning. Experience has shown that the South East USA tends to get rain in the
\nafternoons, so it is advantageous to bias a solar array to make more power in the morning. You
\ncan also split an array into two south east and south west facing arrays. A ratio of 2\u20443 of your
\npanels south east and 1\/3 of your panels south west could spread production out more evenly
\nthrough the day with a bias for recharging batteries in the morning.
\nGrid tie systems should simply be optimized for facing south since their power will be sent into
\nthe grid without the need to recharge batteries or concern for time of production. One may wish
\nto put more panels to the south east to optimize production before afternoon showers roll in.
\nGiven the price of solar panels, it has become economically infeasible to install trackers. The
\ncost of maintenance and the outright cost of a tracking device can be more effectively used on
\nmore panels. If more power is needed over a longer portion of the day, split the panels into two
\narrays.<\/p>\n
In climates where it rains every few days, there is no need to clean your panels. In dry and
\ndusty climates you may need to spray them off periodically. Pollen season absolutely affects
\nperformance and we\u2019ve seen peak power drop by 10% due to pollen and rise again the day
\nafter a rain. Things to keep in mind:1)these panels have to last you 20 years so do not scrub
\nthem or even squeegee them. Think about how many tiny scratches you can accumulate in 20
\nyears. 2)Rain and dew are the best cleaners because they are pure water with no minerals.
\nTap water sprayed onto hot panels will flash off and leave a film of mineral deposits behind
\nreducing production. It is probably best to spray the panels with a garden hose just after dark
\nwhen they\u2019ve cooled and then allow the dew overnight to rinse off the tap water.<\/p>\n
A Photovoltaic (PV) inverter takes 150-600v DC and makes AC power that is synchronized to
\nthe grid. PV inverters come in three varieties: String, Optimizer and microinverter. We\u2019ll cover
\neach in detail.
\nEach input on a PV inverter must be between 120vdc and 600vdc. Please note that when
\npanels are cold they produce a higher voltage and on very cold mornings this higher voltage
\ncould be enough to damage your inverter. For example, your panel\u2019s Voc (voltage open circuit)
\nis rated at 40v and your PV inverter can accept 600v. 600\/40=15 panels seems to be a
\nworkable solution. However on a cold morning that 40v may become 42v and that string is now
\nproducing 630v. The solution is to either calculate the highest Voc voltage for your climate
\nusing information from the cell manufacturer or simply calculate the number of panels in a string
\nso that the maximum system voltage is 525v. This allows a 75v buffer.
\nMost solar panels produce less than 10 amps, so inexpensive 14AWG wire is a good choice.
\nLonger wire runs may dictate a larger gauge wire.
\nSome lower voltage (150vdc) inverters may allow only 3 or 4 panels in series and accept up to 2
\nstrings in parallel (total of 6 or 8 panels). Since the current from each string is likely to be 7 or 8
\namps, two strings will provide up to 16 amps and this will necessitate 12AWG or larger wire.
\nWhen designing a 150vdc system pay particularly close attention to VOC at the coldest
\ntemperature ever recorded for your part of the country. Midnite solar charge controllers have a
\nsafety which disconnects the panels from their equipment is the VOC is too high. It monitors the
\nvoltage and when it is back in range will reconnect the panels. The problem occurs on very cold
\nmornings at exactly sunrise.
\nIn addition to limits on voltage on the inverter, you must also be concerned with maximum power
\nof the array as a whole. It is customary to exceed the wattage of the inverter by 25%. Keep in
\nmind the 1% per year degradation of the panels. After 20 years your panels will be producing
\n20% less power. By exceeding the inverter\u2019s power by 25%, you leave a 5% margin for the
\nfuture. For example if you have an inverter rated at 3.8kw, it is perfectly acceptable to feed it
\nwith 4.75kw of panels. The peak production may max out and flat line but there is no harm.
\nThe inverter won\u2019t be harmed, it simply takes what it can from the panels. Clipping as it is called
\nallows you to produce maximum power earlier in the day and for a longer duration. In the
\nillustration below, you can see that despite the clipping significantly more power is produced as<\/p>\n
compared to an array that matches the maximum inverter power.<\/p>\n
MPPT stands for Maximum Point Power Transfer. To understand what MPPT is requires a
\nbasic understanding of electricity. Power = Volts x Amps. For example 32 v x 7amp = 224
\nwatts. But if the voltage goes to 0 and current is 10amps, power is 0. Likewise if voltage goes
\nto 40 and current is 0, the power is also 0. There is a sweet spot somewhere in the middle.
\nMPPT is a method for finding that spot.
\nYou can demonstrate this technique with a garden hose. Pressure is equivalent to voltage and
\nflow in gallons per minute is the equivalent of current. Placing a 1\u20444 turn valve in the end of a
\ngarden hose allows you to vary from no flow to full flow. If there is no flow, water pressure
\n(voltage) is maximum. If the valve is opened fully, there is full flow and no pressure. By varying
\nthe valve (or your thumb) on the end of the hose) you can quickly find the spot that sprays the
\nmost water the furthest. This is what MPPT does.
\nDepending on the sun\u2019s intensity and panel temperature that \u201csweet spot\u201d may move around
\nfrom moment to moment. Some inverters move their target panel voltage around and see if the
\ncurrent goes up or down. This method is called perturb and disturb. Some inverters cease
\nproduction for a second and sweep all possible voltage \/ current combinations every 5 minutes.
\nIf you see a charge controller or PV inverter with a far too low price, it will probably be PWM
\ninstead of MPPT. PWM simply pulses the power from the panels to your battery and does not
\noptimize for peak power production. They are very inefficient and are to be avoided on all but
\nthe smallest systems.<\/p>\n
A string inverter simply accepts a \u201cstring\u201d of solar panels and typically has 3 inputs. You can
\nconnect three strings of panels to one inverter (West, South, East for example).
\nInverters in this category are SMA Sunny Boy, Schneider, SolArk. As noted earlier, these
\nstrings are particularly at risk for panel shading. One cell of one panel shaded can stop
\nproduction of the entire string.
\nOptimizer Inverter
\nSolarEdge makes inverters which use optimizer modules on the back of each panel. These 4\u201d
\nsquare devices connect to the + and – leads of the panels and then using a second pair of wires
\nform a string of optimizers. The advantage here is that in case of shading any underperforming
\npanel can be switched out of the string automatically. Each optimizer can report each panel\u2019s
\nproduction so if you have a bad panel the SolarEdge app can show you which one it is and
\nwhere it is located on your roof.<\/p>\n
A microinverter takes the 40v from a single panel and makes 240v AC directly on your roof. It
\nmounts behind each panel and connects to a 240v cordset run across the roof. The advantage
\nof this system is that 100% of the electronics are on the roof and do not have to be mounted on
\nthe walls of the structure and the system is not affected significantly by shade. Enphase and
\nMagna Power make these types of systems. They are generally not acceptable for off-grid use
\nbecause 1)enphase does not use a standard method for output limiting, 2)magna energy uses
\nare proprietary method of output limiting. This will be further explained in the off-grid chapter.<\/p>\n
So far, we\u2019ve analyzed household electrical usage, speced a system, placed panels on the roof
\nand selected a PV inverter. All that is left is to actually connect it to the grid.
\nDespite all inverters adhering to UL 1741 and not making power when the grid is down, all
\nelectric utilities require a lockable disconnect. This is so a lineman can ensure that your system
\nwill not back feed the grid if line maintenance is being done.
\nYou will also need a new electric meter. Modern \u201csmart\u201d digital kWh meters are programmed to
\ncount upwards no matter what. This means that if you simply install a solar system on your
\nexisting meter, your production will be counted as usage despite the meter running backwards.
\nIf you are to backfeed the grid, you need to coordinate and get approval from your local power
\ncompany and a \u201cnet meter\u201d installed.
\nA typical install involves turning on the system for about 5-10 minutes to make sure it is working
\nand then shutting it off until the power company can come install a \u201cnet meter\u201d. It typically takes
\n1 to 3 weeks for the power company to send an employee out to swap your meter.
\nResidential service is generally limited to 20kw back feed. Some electric companies will cease
\napproving new solar system installs once they reach 2% of system capacity. The reason for
\nlimiting solar installs is 1) they are in the business of selling power and 2) at mid day the grid
\ncould potentially be flush with power causing them to not need to run a generation station.
\nSince these stations take 10\u2019s of minutes to spool up, they limit solar contributions to the grid to
\na number they can compensate for if the solar were to suddenly drop. Hawaii has exactly this
\nproblem and their island grid is so unstable that the inverter manufacturers use an entirely
\ndifferent set of specs for that grid. The problem is called the duck curve problem. As solar
\npower provides significant portions of electrical demand, the power company shuts down
\ngenerators and can\u2019t restart them quickly enough as the sun sets and people begin cooking
\ndinner.<\/p>\n
This is why megawatt scale grid battery backups are so important in recent years. The point of
\nthe battery is not to run the grid but to smooth out supply and demand – to buy them time to
\nspool up a generating station. Power companies buy and sell power from neighboring grid
\noperators and the ability to fill in demand with a battery while a generator is spooling up instead
\nof buying power at an expensive demand price may represent a significant cost savings.
\nSome power companies exercise a \u201cpeak demand\u201d fee on residential customers for both
\nconsumption and production. For example, if your peak usage goes over 10kw for one second,
\nyou will be charged a demand fee for that month. In the case of Duke Progress Energy, the
\nbasic meter fee is $9.83\/month plus usage. But if you exceed the demand threshold they will
\nbill you an extra $8 for the month. This demand fee \/ penalty also applies to production. If your
\nproduction or usage goes over 10kw you\u2019ll also get dinged for the demand fee. If your
\nproduction exceeds your usage and your bill would have been $0, you will still receive a bill
\neach month for $10-$18.
\nThat\u2019s about it for grid-tie systems. In a nutshell your goal is to run the meter backward enough
\nto offset your electric bill.<\/p>\n
When most people think of an off grid system, they picture a cabin in the woods with no
\nelectrical service for miles. While this is possible, the reality is that the grid reaches nearly
\nevery residential address. Being off grid is not a matter of availability, it is a choice.
\nIn an off-grid system your goal is to run your house when and if the grid isn\u2019t present. For some
\nit may be during a storm when the power is out for a few hours, or it may be to run your house
\nfor a week in the aftermath of a hurricane.
\nOff-grid systems don\u2019t have to be completely off grid all the time. If the grid is present, they can
\nsell power just like a grid tie system.
\nSome may even measure power usage inside the house and never export power but
\nseamlessly import power if needed. These are called net-zero systems.
\nVery often an off grid system is not sized to run the entire house. It is common to create a
\ncritical loads subpanel and relocate certain circuits in the house to that panel. Well pumps,
\nceiling fans and lights come to mind.<\/p>\n
If the grid tie system all you needed to be concerned about was the amount of power produced
\nin a year matching your annual usage. In an offgrid system you must concern yourself with
\nmaking sure your inverter can supply your peak demand and how many days of batteries you
\nhave.
\nThe calculations for off grid include peak usage (ie how many appliances on at the same time)
\nand hours or days of reserve power. Peak usage wasn\u2019t of any practical consideration in a grid
\ntied system because most homes are served by 50kW (ie 200 amp service). Peak power will be
\nmeasured in kW. For example running an 1.8kW hair dryer and a 1.2kW microwave oven at the
\nsame time results in a peak draw of 3kW. In this case you must buy an inverter that can supply
\nmore than 3kW or choose to not run both of those loads at the same time.
\nTo illustrate the difference between kW peak and kWh, consider the following: You run the
\n1.2kW microwave oven 3 times a day for 5 minutes and that is 1.2kW x 1\u20444 hour (15 minutes
\nusage) = 0.3kWh per day. On the other hand, if you use a 25w reading lamp for 12hours, that
\nis also 0.3kWh per day.
\nYou will need an inverter that can supply peak power and a battery that will power you for your
\nanticipated number or days or hours.
\nA fair analogy is that of water supply. You need to size pipes to allow the peak water flow and
\nyou need to use a water tank that holds enough water for a day\u2019s usage.<\/p>\n
Another consideration is the idle power usage on your inverter. Larger inverters at idle will
\ninternally consume more power than smaller inverters at idle. In the case of a 4kw inverter, the
\nidle power is about 52w. This may not sound like much but over a 24 hour period that is
\n1.25kWh. If, for example your battery consists of 8 golf cart batteries with 5kWh of energy,
\nyou\u2019ve just used 25% of your battery to keep your inverter humming all day. Consider this as a
\nwater tank with a slow leak.
\nThe solution for high idle current is to simply switch the inverter off when you aren\u2019t using it or
\nuse a large and a small inverter to run different loads in the house. For example, you may run
\nthe microwave and hair drier on a 3kW inverter that you shut off when not in use and use a
\n300W inverter to run your PC and a reading lamp which stays on all day.
\nThis may be fine for a small cabin, but in a normal home, you would simply size the inverter to
\nrun your peak loads and specify a battery and solar array large enough to cover the parasitic
\nlosses.
\nAnother solution is to interlock high demand loads so that two cannot run at the same time. For
\nexample you can interlock your microwave oven with your well pump. A Dwyer current sensing
\nswitch MCS-111050 can be used to send when an appliance is running and pull in a relay that
\nprevents another device from running.
\nYet another solution for a mixed grid system is to move high current devices to a load panel that
\nis supplied by grid only. For example, it is possible to run a heat pump off grid but running the
\nheat strips is not a good idea. You can easily separate the heat strips from the electronics and
\nblower within your heat pump\u2019s inside unit and feed them from grid only. In the author\u2019s case,
\nthe heat strips are tied to a grid only panel. Additionally, the contactor that switches them on
\ncan simultaneously activate gas logs. The idea here is that if the grid is down and the heating
\nsystem needs heat strips it can fire up the gas logs.
\nThe author of this article is presently writing an application in Node Red running on a Raspberry
\nPI to do load management in the house. For example, if the electric cars are charging and the
\nthermostat is calling for heat, the cars will be commanded to slow or stop charging before the
\nHVAC system is activated. If other loads in the house are too high, the HVAC system may wait
\nuntil the household usage drops below 3kw.
\nAnother way to deal with high capacity requirements while keeping idle power draws low is to
\n\u201cstack\u201d inverters. Many brands of inverters can be linked so they stay in phase (in sync) with
\neach other and only switch on as many inverters as power is needed.
\nMagna Sine inverters are linkable in such a way that one 4kw inverter stays on as the master
\nwith up to 3 additional inverters in hot standby mode. As loads increase in the house, the other
\ninverters are automatically brought online.<\/p>\n
Most other inverter brands simply stack the inverters in parallel for more power and all are on
\nidling 100% of the time.
\nBe aware that 240v inverters not meant for the US market will NOT be split phase. They will
\nproduce 240v instead of the 120\/240v used in US households.
\nFood for thought: It is entirely possible to use a 240v inverter to run 120v appliances by using a
\nstep down transformer. This has the advantage of not having to worry about balancing 120v
\nloads on the two legs of a split phase system. Imagine if the microwave and hair drier are both
\npowered from the same leg of 120v. It may be possible to overload that one leg while the other
\nhas no load at all. By taking the entire 240v feed through a stepdown transformer to 120v. To
\nuse the step down 120v method, use 240 from the inverter in a conventional panel box to run
\nstoves, ovens, well pumps and also a step down transformer. The 120v output of the step down
\ntransformer can then be run to a 120v only panel box for all 120v loads in the house.<\/p>\n
SMA Sunny Island inverters for the US market make 6kw of 120v and so must be added in pairs
\nto make 120\/240 split phase. To make 120\/240 you must use either two or four Sunny Islands
\nmaking either 12kw or 24kw.
\nAnother consideration is the system voltage. Very small inverters typically run at 12v. Mid sized
\ninverters between 1kW and 4kW tend to be 24v. Inverters larger than 4kw are generally 48v. In
\npractical terms, if you intend to grow your system beyond 4kw, you should strongly consider 48v
\ninverters even if your system only needs 2kw today. The issue is that current gets so high it
\nnecessitates very large cabling. For example, 4kw at 24v is 166amps and requires AWG 3\/0
\ncable. 4kw at 48v requires AWG 3 cable. More on battery voltage later.
\nBeware of cheap chinese inverters which claim some high wattage but don\u2019t back it up with
\nspecification. For example, the Sunny Island 6kw inverter says in the specs it can do 7kw for 3
\nminutes, 8.4kw for 1 minute and 11kw for 3 seconds. A 4kw Aims inverter struggled to provide
\n3.5kw.<\/p>\n
Off-grid systems can be AC or DC coupled. A DC coupled system uses a solar powered charge
\ncontroller to charge a battery and the inverter then draws power from the battery to power the
\nloads in your house. This is what most people first imagine how off grid solar works. The
\nbattery inverter is the primary power source for all loads in a DC coupled system. DC coupled
\nsystems can sell power, but 100% of the power sold must first go through the batteries and<\/p>\n
there may be a 15% loss in efficiency in\/out of battery. DC charge controllers tend to cost more
\nthan their PV inverters.
\nIn the picture below, the solar panels feed a 48v charge controller (small yellow box) that
\ncharges the batteries. The Sunny Island inverter (larger yellow box) can draw off the batteries
\nand also recharge the batteries when the grid or generator is present.<\/p>\n
DC charge controllers fall into two categories: expensive 150v or 200v units and really
\nexpensive 600v units. Midnite Solar and Morning star make very well respected DC charge
\ncontrollers.<\/p>\n
An AC coupled system works in conjunction with PV inverters and simulates the grid to keep
\nthose PV inverters running the loads in your house. In the day time the primary source of power
\nfor your appliances in the solar PV inverters and the battery inverter is basically there for backup
\nsupport if power needs exceed solar production at that moment. Another advantage of an AC
\ncoupled system is that of peak power. If your battery inverter is capable of producing 12kw and
\nyou have 12kw of solar PV inverters, then if the sun is shining, you actually have 24kw of power
\navailable to the house.
\nThe central point of the AC coupled system below is the Sunny Island inverter. The battery
\ninverter becomes the gatekeeper for the grid interface, a battery charge controller and also
\ncontrols the PV inverters.
\nIn the picture below the red Sunny Boy inverter (right) takes DC from the solar panels and
\nmakes AC that can run appliances directly and simultaneously the Sunny Island (left) can use
\nthat power to charge the batteries.<\/p>\n
First and foremost, the inverter provides a 240v \u201clifeline\u201d signal that is very stable to fool the PV
\ninverters into continuing to run. You may recall from the previous chapter in this article that UL
\n1741 requires inverters to shut down for 5 minutes if the grid drops. The battery inverter
\nsimulates the grid well enough to keep the PV inverters operational. Should the sun go behind
\na cloud or the sun set, the 240v lifeline signal will begin to produce power to run your house.
\nThe battery inverter watches the quality of the connected grid and will sever the link to the grid if
\nit is out of spec. Just as with the UL 1741 rules, the battery inverter will wait for 5 minutes of
\nstable grid power before reconnecting. It\u2019s job is to separate from the grid and keep your PV
\ninverters running.
\nAnother function of the battery inverter is to recharge the batteries. Typically the maximum
\ncharge current to a battery can be in excess of 120amps DC. These inverters have many
\nsettings for bulk, absorption and float charge voltages. In many cases these voltage settings
\ncan be adjusted to be suitable for 24 or 48v lithium batteries.
\nThere is a special case in an off grid system where the grid is disconnected and the batteries
\nare full and demand is low. In this case the PV inverters must be throttled back. There are two
\nways to do this: 1)proprietary communications cable, 2)Frequency Shift Power Control.
\nIn the case of frequency shift power control, consider first that the normal line frequency for
\nNorth America is 60hz +\/- 0.5hz. If a battery inverter sees the frequency higher than 60.5hz or
\nlower than 59.5hz, it assumes the grid is unstable and will disconnect. Once disconnected from
\nthe grid, the inverter will then operate the solar PV inverters between 55.5hz and 62hz. The
\nmagic happens between 61 and 62hz. PV inverters complying with this method will throttle
\nback to making no power at 62hz but will remain in \u201chot standby\u201d. If the frequency drops any
\nlower than 62hz, they will immediately make up to 100% of the power they can.<\/p>\n
Why 62hz? It goes back to gas powered generators. When you put a heavy load on a
\ngenerator, it will slow down below 60hz. When there is no load on a generator it will speed up to
\nit\u2019s preset idle of 62hz. So a generator making 62hz doesn\u2019t need any additional power and a
\ngenerator making just below 60hz needs all the additional power it can get.
\nhttps:\/\/aeesolar.com\/wp-content\/uploads\/2018\/01\/AC-Coupling-and-Frequency-Shifting-DC2018
\n.pdf
\nSMA Sunny Island inverters maintain a count of the amount of time the frequency was shifted
\nabove 60hz in a 24 hour period and will run the system at a frequency lower than 60hz for an
\nequivalent amount of time. The purpose of this is to ensure that clocks on your microwave and
\nstove don\u2019t run fast. Every 24 hours they will be run fast or slow as necessary to reset to the
\ncorrect time based on 60hz AC power.
\nAnother method for using excess solar power when demand is low is to heat hot water. Even if
\nyou use an instant hot water heater you can benefit from solar hot water. Simply install a 40
\ngallon hot water heater inline ahead of your instant water heater. The warmed water will offset
\nthe energy needed to heat your water to the desired temperature. Three 255w solar panels or 6
\n255w solar panels in series can directly drive a 120v or 240v water heater element. You need
\nonly to swap the thermostat with a thermostat suitable for switching DC such as one sold by
\nMissouri Solar https:\/\/mwands.com\/water-heater-thermostat . DO NOT use the standard
\nthermostat included with the water heater. And always ensure that the emergency popoff
\nthermostat remains wired into the circuit.
\nMany battery inverters have programmable relay outputs that can indicate battery State of
\nCharge and these can be used to divert solar panels to other loads.<\/p>\n
Most people will begin a system with 6v golf cart batteries. If you treat them right and never
\ndischarge below 50% they will last about 5 years. You\u2019ll have to add distilled water periodically.
\nA 6v battery consists of three 2v cells and over time each 2v cell may become imbalanced. The
\nmethod used to \u201cbalance\u201d lead acid batteries is crude… you overcharge them until the weakest
\ncell is at the same voltage as all the others. Typically, each cell is charged to 2.6v (63v on a 48v
\nbattery). This causes a great deal of heat and the water boils out of the electrolyte producing
\nhydrogen gas. This gas must be vented and water level should be checked in each cell.
\nThe plates of golf cart batteries aren\u2019t particularly thick and each equalization cycle erodes them
\na little. Eventually all that lead precipitates to the bottom of the battery and shorts out the
\nplates. L16 batteries have much thicker plates than golf cart batteries and can take more
\nabuse. The most robust lead acid battery of all is a forklift battery. In forklift service they take
\n300amp loads continuously and last 5 years. In residential service, they rarely see high loads
\nand can last 15 years. A 48v forklift battery can have a capacity of 45kwh and weigh 2600 lbs.
\nThey do require distilled water fill touchups once a month and they do require proper ventilation.
\nThe picture below shows a SMA 3.8kw Sunny Boy, a 11.4kw Solar Edge, two SMA Sunny
\nIslands and a 48v 45kWh forklift battery. The battery was later covered with a ventilated
\nenclosure.<\/p>\n
AGM batteries contain the boiled off water and recycle it back into the cell. They require no
\nmaintenance after the equalization (balancing ) cycle. They also only last about 3 years.
\nLithium batteries
\nThere is nothing magical about lithium battery charging. Unlike a lead acid battery there is no
\nneed for an absorption or bulk charging cycle. You simply push electrons into them and stop
\ncharging when full. The problem comes when you approach 4.2v per cell. Please recall from
\nabove when we discussed balancing lead acid cells by over charging them and boiling off water.
\nLithium batteries cannot boil off water so they can only make heat… and catch fire.
\nA battery is comprised of cells in series. Lithium batteries are routinely described as 6s or 14s.
\nThis means 6 or 14 cells in series. Since each cell is 4.2v that nomenclature tells us it is a
\n25.2v or 58.8v battery. In the lead acid example above we discussed one cell being weak. The
\nsame problem can exist for lithium. Imagine a string of 6 cells where one is weak. 5 cells are
\ncharged to 4.2v and one cell is at 3.8v. If our charger continues to charge these cells to 25.2v
\nwe will have 5 cells with 4.28v and one with 3.8v. Chances are good that one of those 5 will
\ncatch fire. The solution is a battery management system.<\/p>\n
A battery management system will measure the voltage on each cell and stop the charging
\nprocess when any cell reaches 4.2v. Some battery management systems will use resistors to
\ndrain the cells with higher charge until their voltage matches the weakest cell at which time
\ncharging can resume. Other more sophisticated systems like Batruim can \u201cshuffle\u201d excess
\nelectrons between cells rather than simply bleeding off thru a resistor.
\nOn the other end of the lithium scale we have to worry about over discharging the cells. Just as
\nwith charging, the battery management system also watches for any cell to get below 3.2v and
\nstops all drain.
\nIf you ever run a tool battery down too low, it can often be \u201cjump started\u201d by connecting it to a
\nsimilar battery to allow it to charge to a level that the charger can recognize as valid and being
\ncharging.
\nThe simplest lithium charging system consists of several 40v solar panels in parallel connected
\nto the battery through a large relay. The panels put out 40v with no load but when connected to
\nbatteries will load down to the battery voltage. In one such system, the designer has 16 panels
\nin parallel producing 112amps. There is a voltage sensing relay on the battery and when it gets
\nto 25.2v, it will disconnect the panels from further charging. This system has no BMS and
\nserves only as a proof of concept. A California man claims to have run his house off this system
\nwith no BMS for nearly a year and only sees 9mv (0.009v) difference between any two cells. I
\ncannot enphasize enough that there must be some method of detecting when any cell is over
\n4.2v and stopping charging immediately.
\nMany people will attempt to use Tesla or GM Volt lithium batteries. The problem with these
\nbatteries is that they are 6s or 12s. An adjustment needs to be made to the inverter\/charger to
\nmake sure these cells never charge higher than 25.2v or 50.4v. Additionally most inverters will
\nshut off for low battery at 21v or 42v. Since 6s or 12s are good down to 18v or 36v, a lot of
\ncapacity goes untouched. The solution is to create at 7s or 14s battery.
\nFor a 24v system the only practical way to get a 7s battery is to make them from individual
\n18650 cells. There are many groups of facebook which cater to 18650 battery packs such as
\nDIY powerwalls.
\nFor a 48v system you can make the battery from 18650 cells or from 7 Nissan Lead \u201cspam
\ncans\u201d. Since a Leaf car battery comes with 48 spam cans, buying one additional spam can from
\nebay can make a 7s and 7parallel configuration which is very well matched to 48v lead acid
\ncompatible inverters and chargers.
\nTesla makes the famous Power Wall. It uses lithium batteries and an inverter in the same box.
\nWhile they certainly work and are maintenance free, a handy person can home brew a system
\nthat has 9 times the capacity for the same money.<\/p>\n
Generator tie-in
\nIt can be quite expensive to buy a 200amp transfer switch for your whole house. A more cost
\neffective way is to install a generator back feed breaker in the top left or right column in your
\nmain panel and interlock that breaker with the 200amp main breaker. You can make a sliding
\ngate yourself or buy one from https:\/\/www.geninterlock.com\/ .
\nHere is an interlock used to select grid or inverter power in the author\u2019s home. This facilitates
\nbypassing the inverters if they need service.<\/p>\n
It is also worth mentioning that many inverters such as SMA Sunny Island have a mode switch
\nthat is usually tied to the transfer switch used to swap in a generator. This mode switch
\nchanges certain parameters in the battery inverter and signals to the inverter that generator start
\nis available should the batteries get low. The settings in the inverter generally relax the
\ntolerance for frequency and voltage changes.
\nCheap Chinese inverters such as Aims claim to have a generator start function but it does not
\nwork well. The gen start will trigger when the batteries are critically low and your generator will
\nstart. After 30 seconds the inverter will switch the AC input on and begin charging the battery.
\nWithin a few moments the battery voltage will rise above 24 or 48 volts and the inverter will
\nrelease the generator start output. The problem with this is that the batteries didn\u2019t really charge
\nin that short time and now that the generator isn\u2019t running, the batteries are in an even lower<\/p>\n
state of charge. This rapid cycling will continue until the batteries are depleted. The solution is
\nto buy a timer from ebay that once triggered keeps the generator start signal asserted for 1 to 3
\nhours.
\nAnother problem arises when shutting down a generator. As the generator is shutting down and
\nslowing, it\u2019s on board voltage regulation spike the armature current to keep the output voltage
\nup. This can burn up the brushes. It is why generator manufacturers advise to disconnect all
\nloads when shutting them down. Another side effect is that the erratic behavior of the voltage
\nregulation results in a reactive power alarm on many solar PV inverters. The solution here is to
\nuse a 12v contactor in line with the gen start signal that releases as soon as the generator start
\nsignal is lost. In this way the generator is electrically disconnected at the moment the gen start
\nsignal is lost.
\nYet another problem with generators on an off grid system is the interaction with the PV
\ninverters. Although the frequency shift method is supposed to be compatible with generators,
\nthe PV inverter companies advise against connecting to a generator. The generator
\nmanufacturers also advise against running solar at the same time as the generator. The
\nsolution is to interlock the generator start signal with a relay that disconnects the PV inverter
\nbefore the generator can even start. Elkhart \/ Esco (574) 264-4156 sells a DPDT 50 amp
\ncontactor with an auxiliary switch part number 21082-84. The generator start signal can pull in
\nthe contactor coil and if the PV inverters are connected to 240v mains via the NC (normally
\nclosed) contacts, they will be disconnected. The auxiliary switch can then be used to then
\nactivate the generator start directly. This ensures that the PV is disconnected before the
\ngenerator can even start avoiding any warranty voiding back feed power issues.<\/p>\n
Would you buy a car from a buy here pay here car lot? Neither would I. So why would you
\nentertain a solar company that touts \u201cno money down\u201d or \u201cwe\u2019ll pay you to install solar\u201d?
\nHere\u2019s how the scam works: In South Carolina, you can get a tax credit for 25% state and 26%
\nfederal. A solar company will sell you a system and finance it into two loans. 51% of the price
\nwill be in one loan and 49% in the other. The 51% loan will be a short term with a balloon
\npayment that must be satisfied when you file your taxes the following year. The problem arises
\nwhen, due to your financial circumstances, you aren\u2019t due a large enough refund to cover the
\nloan payoff. The solar tax credit is a refundable credit meaning that you can only get a refund
\non taxes paid this year. If you can\u2019t claim enough tax to offset the payoff on that loan you must
\ncarry over the excess credit until next year. This is the crux of the problem. The loan is due
\nbefore you can file your taxes the following year.
\nThe other portion of the loan is structured to be paid in 20 years. That loan will have a lien filed
\nagainst your home that must be satisfied before closing if you sell your house. That loan can be
\ntransferred to the new homeowner, but ask yourself… if you were buying a house that came
\nwith solar would you volunteer to assume payments on a loan someone else was responsible
\nfor?
\nIf a system is financed, FHA guidelines do not allow it to be included in the appraised value of
\nyour home. A paid-for system will be included in the appraisal. In short a system with a lien on it
\nis a liability and a paid off system is an asset.
\nPaying for solar doesn\u2019t always have to be a bitter pill. Duke\/Progress in North Carolina often
\noffers a cash rebate of $1\/PV watt if you use their preferred installers. In the case of a recent
\ninstall, the homeowner received a check in the mail for $16,500 about 30 days after their system
\nwas commissioned. Here is an example of an affordable system. The homeowner installed a
\n16.5kw system for $48000 cash. 30 days after the install Duke Energy sent a rebate check for
\n$16,500 bringing the system price down to $31,500. At the time of this install the combined SC
\nand Federal solar tax credit was 55%, so at the next income tax filing the homeowner received
\n$26,400 in tax refund. This was actually split over two years as a carryover. This meant the
\nout of pocket cost was $5100. The electric bill had been $1850 per year. The system made
\nenough power to sell $700 back to the power company the first year, so $2550 was offset from
\nthe power bill. Payback was two years on the nose. Typical payback is 6-8 years.
\nWhen talking to solar installers you need to discuss kWh per year of production and dollars per
\nwatt installed. A good starting point until recently was $3\/PV watt for a grid tied system. Many
\nsolar installers want to sell systems at $4 or even $5 per watt and offer to replace the light bulbs
\nin your house with LED or to blow insulation into your attic in addition to the solar install. Why
\nwould you pay extra to have someone do something you could do yourself? Recently Tesla
\nupset the industry by offering turn key grid-tie systems for $1.50 per watt. Keep in mind that<\/p>\n
cash is king… if you need the installer to get financing for you he\u2019s going to charge more per
\nwatt. If paying cash you can bargain with him to stay nearer to that $1.50 or $2 per watt price.
\nYou should expect the salesman to measure your roof area and provide a layout of the panels.
\nThis layout should include 3\u2019 easements around three sides of the panels and 18\u201d on each side
\nof a valley or dormer. If they want to skip a panel due to a plumbing vent, offer to have a
\nplumber install an air admittance valve in your attic and remove the vent pipe. The hole can
\nthen be repurposed to feed the wires into your attic.
\nThe salesman should provide you with a model of the estimated production per year that takes
\ninto account your roof pitch and the direction of your roof layout. For example, a 16.5kw system
\nshould produce 30,714kWh per year (16.5 x 5.1hours per day x 365days) but if the roof pitch is
\nnot optimal, it will produce 23,000kWh per year. Make sure you check his model carefully. Do
\nnot let them talk about how many dollars cheaper your power bill will be… keep the
\nconversation around kWh used and produced.
\nA reputable solar company will guarantee their estimated production for a number of years after
\nthe install. Fly by night companies will sell you panels they know won\u2019t produce due to shade or
\norientation issues.
\nMake sure that the solar company uses their own installers and does not job-out to
\nsubcontractors. There was a case in South Carolina where the salesman bid a job for 25
\npanels but did not observe the easement requirements. When the contractor arrived on site to
\ndo the install, he realized that they could only fit 21 panels on that side of the roof. His solution
\nwas to convince the homeowner to install the panels on the north side of the roof because it
\ndidn\u2019t have dormers. The system has never made any significant amount of power and the
\nhomeowner is stuck with an electric bill and a loan payment. Later there were leaks and the
\nsales company directed the customer to contact the subcontractor. The sub wouldn\u2019t talk to
\nthem because they hadn\u2019t paid him directly and the sales company said it fell on the contractor.
\nRound and round they went. Long story short, use a company that employs its own install crew.
\nGet your solar installer to give you access to daily production reports from your system. All
\nsolar inverters sold today have the ability to connect to your home wifi lan. You can check
\nproduction daily using an ipad. Waiting for the next electric bill can allow a problem to go on far
\ntoo long.<\/p>\n
The talking points in this article were meant to familiarize you with the concepts and terminology
\nused in residential solar systems. This is by no means detailed enough to design a system but
\nshould be treated as a primer.<\/p>\n","protected":false},"excerpt":{"rendered":"
Powering your house from the sun. In 2016 we sold a lot of mature timber<\/p>\n","protected":false},"author":5,"featured_media":641,"comment_status":"open","ping_status":"open","sticky":false,"template":"","format":"standard","meta":{"_jetpack_memberships_contains_paid_content":false,"footnotes":"","jetpack_publicize_message":"","jetpack_publicize_feature_enabled":true,"jetpack_social_post_already_shared":false,"jetpack_social_options":{"image_generator_settings":{"template":"highway","default_image_id":0,"font":"","enabled":false},"version":2}},"categories":[24],"tags":[],"ppma_author":[359],"class_list":["post-603","post","type-post","status-publish","format-standard","has-post-thumbnail","hentry","category-guides"],"yoast_head":"\n