HOW TO SIZE A PV SYSTEM
The owners of Signature Solar have operated a 10 person, 5000SF off-grid complex in northeast Texas for over 8 years. We have installed over 150 solar electric systems totaling over 3 Megawatts of power, and we are happy to help those who are interested in going solar. We know that solar is an important investment decision for your family, so we have written this to give you as much information as possible to help you make an informed decision.
You pay for electricity in Kilowatt-Hours (KWH). One Kilowatt-Hour is the use of 1000 Watts for one hour. The first step in sizing a solar electric system is to estimate the total number of Kilowatt-Hours (KWH) you use each month. Your electric bill should show this. If you don't have an electric bill for this location yet, estimate. Your bill should state how many KWH you used for the month. If you cannot determine your monthly KWH use from your electric bill, you probably know how much your bill is each month. First, subtract out fees that are not related to the amount of power you use. Delivery charges are not fees if they are related to the amount of power you use. Try to find out what your utility charges for electric power in dollars per Kilowatt-Hour ($/KWH). If you can't find this on your bill, you can use the national average cost of electric power, which is 11.3 cents/Kilowatt-Hour or $0.113/KWH, to get a rough idea. Divide the dollar amount of the use-dependent part of your monthly bill (without the fees) by the cost of electricity, and this will tell you about how many KWH you are using each month.
For example, if you pay $200/month of actual energy charges and you pay the national average rate, you use about 1770 KWH/month. Divide your monthly use by 30 to get average daily use, in this case 59 KWH/day. Most American homes use between 50 and 75 KWH/day. You should average your monthly use numbers throughout the year to get a good average number as your design basis, or you can deliberately design your system based on your maximum monthly use. Then, if you are grid-tied, multiply this number by the percent of your current use that you want solar to replace. In the example above, if you are grid-tied and want solar to replace 75% of your monthly use, you need enough panels to produce about 1328 KWH/month or 44 KWH/day on average. Off-grid customers must supply 100% of whatever appliances they are trying to power; these circuits may be wired to a sub-panel that is separate from their main grid-tied panel.
Next, go to a website called pvwatts.nrel.gov, produced by the National Renewable Energy Laboratory (NREL). Your goal is to determine how many KWH will be produced in one year by a 1 Kilowatt (KW) array of solar panels in your location, with your type of racking, tilt, direction, etc. Once you know this, you can divide your expected annual demand by this number to determine how many Kilowatts of panels are required to exactly replace your annual demand, IF life worked according to averages. Remember, PVWatts data is based on averages. Solar production on a cloudy day is about a tenth of what it is on a sunny day. Important Note: Do not confuse a “Kilowatt” of solar panels with a “Kilowatt-Hour” of electric power. 1 Kilowatt (KW) of solar panels is 1000 Watts of panels. For example, if you have 4 solar panels each rated at 250 Watts, you have 1000 Watts or 1 “Kilowatt” (KW) of solar panels.
With PVWatts you can change one variable, such as tilt or azimuth, and immediately see its impact on your total annual production. Each time you make a change on the SYSTEM INFO page, you can click the large orange arrow on the right labeled “Go to PVWatts results”, then from RESULTS you can click the large orange arrow on the left labeled “Go to system info”. You can go back and forth between these two pages as many times as you like, each time noting how a change in one parameter will affect production. If PVWatts has a glitch and will not let you return to the SYSTEM INFO page, click “Change Location”, retype your zip code and start again.
So go to PVWatts, type in your zip code and click the right arrow (or hit Enter). PVWatts now shows your location on the map and has already accounted for the climate data in your area, such as sun intensity, number of rain days, etc. Next, click the large orange arrow on the right labeled “Go to system info”. Here is a picture of the SYSTEM INFO page:
Change the first item, DC System Size in KW, to “1”. Next, accept the default value of “Standard” for “Module Type”.
For most people, the next item, “Array Type” should be Fixed (open rack) or Fixed (roof mount), depending on where your panels will be mounted. FYI, mechanical tracking systems increase solar production by only about 20 percent, so consider this before you incur the additional investment and maintenance cost. Comment: If you choose to mount panels on the roof of an occupied building, the 2017 National Electrical Code (NEC) requires that rapid shutdown devices be located close to each panel on the roof, increasing your system complexity and cost, usually by about $1000. This requirement does not apply to non-occupied buildings such as barns, sheds, garages, carports, or ground mount solar racks, providing they are not attached to an occupied building. If they are an option for you, ground mount solar racks are easier to install and maintain, stay cooler, cannot cause a leak in your house roof, and can be used for storage.
The next line in PVWatts is “System Losses (%)”. We change this number to 6% for grid-tied systems and 9% for off-grid systems based on our experience. Next, enter the tilt of your solar panel array in degrees. Just pick a number, try it, and adjust it later. A lower tilt increases summer production, and a steeper tilt increases winter production. We often build ground mount racks with an 18 degree slope to maximize summer production for air conditioning in Texas.
The next line in PVWatts is “Azimuth”, which is a numerical way of telling which direction the solar panel array is pointing. 180 degrees is due south, 90 is east, 270 is west, etc. Click the “i” button for more information. Maximum annual production is usually at an azimuth of 180 (due south) or slightly southwest, azimuth 180-200.
You can leave “Advanced Parameters” and “Retail Electricity Rate” alone. If you do look at Advanced Parameters, note that the DC to AC Size Ratio default is 1.2. This point is often misunderstood. The Wattage of each panel is rated when the panel is by itself under ideal laboratory conditions: Irradiance (power coming in) of 1000 Watts/square meter and a solar panel cell temperature of 77 degrees F. In reality, irradiance is about 800 Watts/square meter, cell temperature is about 114 degrees F or more, and electrical resistance increases with temperature and with the number of panels connected in a string (in series). Actual production exceeds 80 percent of rated power for only about 5% of the day. For this reason, solar designers typically use a DC to AC ratio of 1.2 to 1.3 for grid-tied systems. If you are grid-tied, try to design your system to be in this range if possible. This increases inverter efficiency and avoids “clipping” caused by not having enough power from the panels to feed the inverter.
Off-Grid Note: DC to AC ratio does not apply to you because you must design your system to collect enough power to store in batteries to get you through the night. Even with grid power backing up your inverter, you have to size your inverter to meet your highest power demand at any given time. Use the method I am describing to determine your basic system size, then increase the size for contingencies or if you want to store power for more than one day. It's OK to start with more inverter capacity than panel capacity, then add more panels later.
Now click the “Go to PVWatts results” arrow on the right to go to PVWatts results. These are the results for our area:
The number in the middle column at the bottom of the page is what you are looking for: How many Kilowatt-Hours (KWH) you will make during a year for every 1 Kilowatt of solar panels you have, based on your assumptions. If you divide this number by 365, it tells you the number of Kilowatt-Hours per day per Kilowatt of panels (KWH/day/KW of panels). This is also the number of equivalent sun hours at your location. This number is usually between 3 and 6 in the continental U.S., with about 4-5 being most common. Our result is 4.4. If your system ls on a roof, you will need to calculate this number for the south, east, and west sides of the house, and then use a weighted average based on of the number of panels on each side to see what you will actually produce.
Another important point: PVWatts tells you how many KWH you are making during each month of the year. Divide this number by the number of days in the month to see how many KWH/day will be produced by 1 KW of panels during each month of the year. Here in Texas, minimum production (December) is about 59% of maximum production (July). In the extreme northern part of the United States the seasonal production difference is much greater, with December production being about 30% of July production or even less, even so you may want to adjust the design of your system (for example, the tilt) to increase production during months that are especially important to you in your location.
Once you have adjusted the SYSTEM INFO parameters to your satisfaction, go to the RESULTS page and record the number at the bottom of the page in KWH/year/KW of panels. Next, divide your estimated annual demand in KWH by this number. The result will tell you how many Kilowatts (KW) of panels are needed to exactly offset your annual demand for electricity.
It may be easier for you to relate to electric use and solar production on a daily basis. First, divide your annual demand by 365 to get daily demand in KWH/day. You can divide the KWH/year/KW of panels you got from PVWatts by 365 to get KWH/day/KW of panels. If you divide KWH/day by KWH/day/KW of panels, it should give you the same system size you got earlier.
Of course, life does not operate by averages as PVWatts does, so include a reasonable safety factor in your design; we usually choose about 10-20% extra production. From here, you can decide what portion of your annual demand you want to offset with solar. You can start as small as you want, but remember, when you actually implement or expand your system you will be stringing panels together in series to meet an input voltage spec on an inverter or charge controller. It will be easier to make this work together if your panel strings have approximately the same voltage and amperage. Also remember that the residential Solar Tax Credit (Form 5695) is 26% for 2020, 22% for 2021, and 0% thereafter.
If you install a grid-tied system, you will become the electric supplier and your utility company will become your customer. Most of your solar production will be made between 10 AM and 3 PM. Any power you produce that exceeds your immediate house demands (“loads”) at that time will be “exported” to your utility. You need to know how your they will compensate you for this power, and what they will require for the connection. Some utilities offer “Net Metering” based on state or local government requirements. This means the utility will credit your account a full KWH for each KWH you export to them. The credit is then used against KWH you buy from them in the evening, and your bill is “netted out” at the end of each month; this way, they may end up paying you. Find out exactly what your utility requires to qualify you for net metering. Some utilities limit the size of a solar electric system based on past years' bills or other constraints. Also, some jurisdictions require solar panels to be “set back” a certain distance from the ridges and eaves of a roof.
Other utilities offer solar customers a “wholesale buyback”. This means they will only pay “avoided cost” for the KWH your system exports to them, because they still have to pay for the grid infrastructure, grid maintenance, and increased production capacity. For most utilities, solar production exported to them between 10 AM and 3 PM does not reduce their government mandate to supply ALL of an area's electric demands during “peak demand hours” between 4 and 10 PM; in other words, they still need to buy the same size generator they would have bought without solar.
This is why, in many cases, the “export rate” a utility pays you for your excess solar production is only a fraction of what they charge you for the same KWH. Some utilities have negotiated with their governing body to be allowed to reduce their export rate in the future, and others are attempting to do so. They also may limit or restrict the size of your solar electric system or the amount of KWH they will buy from you during a certain period of time. Find out these details in the design stage of your project so you will not have any surprises.
Even if your utility does wholesale buyback, you still have a good shot at zeroing your electric bill. You can see that if, for example, your utility charges you 11 cents/KWH and pays you 4 cents/KWH, you would have to multiply the system size calculated above by 11/4 to match your current electric bill dollar for dollar. Obviously, this will greatly increase the payback time of your solar investment, and that time may increase if your utility is allowed to drop its export rate.
To deal with this, inverters called “Self-Consumption”, “Demand Management” or “Export Blocking” models are arriving on the market. They allow customers to store excess solar electric production generated during the day in batteries, rather than exporting it to the grid. When solar production declines in the afternoon, this type of inverter can supply all of the house loads with power from the batteries, acting as an off-grid inverter for a period of time. It can also provide back up power to certain critical circuits (such as refrigerators, freezers, lights, etc.) even when the grid is down, if these critical circuits have been wired to a separate sub-panel. Many “hybrid” inverters on the market also use batteries to back up critical circuits in this way, but are not able to perform self-consumption from batteries in the evening when the grid is operating.
Actually, having a battery backup of ALL of the circuits in your house in the event of a grid outage, or going completely off-grid, is easier than you think, if your system is designed correctly. This can be done whether or not you are already grid-tied. I will discuss that in a different PDF, but at this point, you know how to size your solar electric system.
Disclaimer:
Any advice given by James Showalter and/or Signature Solar is meant simply as conceptual clarification. It should not be interpreted as a call to action by anyone who is not a licensed, trained and insured solar electrical installation professional.
The equipment sold by James Showalter and/or Signature Solar is designed by the manufacturer to be installed ONLY by licensed, trained, and insured solar electrical installation professionals. We strongly advise the customer to seek the assistance of such a professional to exclusively perform the implementation of any of these products, and we make no warranty of the purchaser’s safety, success of equipment implementation, or compliance with local codes and regulations. James Showalter and/or Signature Solar disclaim all warranties, expressed or implied, including but not limited to, any implied warranty with respect to the accuracy or completeness of the information they disseminate and/or fitness of the materials sold for a particular purpose. Some of the equipment sold may not include a manufacturer’s warranty. No warranty may be created or extended by sales or promotional materials.
The default is 1 If you use 1750 KWH/month from September-May and your use jumps to 3000 KWH/month during the June-August, summer, your annual use is about 25,000 KWH/year. pvwatts.nrel.gov says that in your location an AC-coupled off-grid system using off-grid and grid-tied inverters, with panels on a 22 degree slope will produce about 1500 KWH/year/KW of panels for a south facing roof and about 1250 KWH/year/KW of panels for an east or west facing roof. Averaging these 2 numbers together to get a ball park number gives 1375 KWH/year/KW of panels. Dividing 25,000 KWH/year by this number gives about 18.2 KW of panels required to match your annual use of electricity if everything worked perfectly according to averages. (This is more than the 16.5 KW I said yesterday because I was not accounting for the fact that about half of your roof faces east or west, rather than south).
PVWatts accounts for rain/cloudy days in your area, but as you know life is not average and you can have a string of cloudy weather. You can also have excess demand on certain days. For example, assuming 3000 KWH/month during the summer means 100 KWH/day, but from 6 PM last night until 10 AM today you used 136 KWH, so it's a good idea to build in a safety factor. Basically a 20 KW system will give you a 10% safety factor and a 22 KW system will give you a 20% safety factor, so when you look at the pictures you can eliminate some of the panels based on aesthetic or practical considerations. But remember, the bulk of your solar production will take place between 10 AM and 3 PM. When you are off-grid, you must make enough during these hours to supply your house loads during these hours and store enough excess in your batteries to take you through the night. For off-gridders, panels represent battery charging capacity, so additional panels increase your ability to charge your batteries quickly when the sun comes back out, thereby increasing your safety factor.
One of the best things you can do if you are considering off-grid is to make an energy budget so you know how many Watts you are using during each hour of the day. Try to eliminate energy-intensive activities or shift them to the daylight hours, thus saving batteries. If you do need to run a generator occasionally, get some battery chargers that can charge your batteries directly. Do NOT try to pass generator AC through an off-grid inverter, even if the inverter says you can do this. A pure sine wave off-grid inverter makes the cleanest power, followed by the grid. Generator power is usually "dirty" (non-standard) and can damage appliances as well as your inverter. It is better to get a good quality, transformer-based inverter, buy a few backup boards for it, and charge your batteries directly from the generator. As long as the battery bank is charged, the off-grid inverter will keep producing power. More about inverters later.
Here are our equipment options. All equipment is NEW and is eligible for the solar tax credit. All panels contain manufacturer's labels. Spec sheets are attached. "Mono" means monocrystalline cells, which is the preferred type.
Not surprisingly, you can get a little higher total capacity with the smaller 60 cell panels. Remember that it is not necessary to put every one of these panels on your roof. This is how I would size you system:
72 Cell Panels:
Grade A Canadian Solar 385W Bifacial Mono, 79.6" x 39.1" x 1.2", 56.7 lbs. 30 year manufacturer's warranty. 35 panels/pallet. $0.45/Watt or $173.25/panel.
"Bifacial" means the panels have cells on both front and back, with a bus bar in the middle. They produce more than rated power because of UV rays reflecting onto the back of the panel. We estimate excess production at 5% for a shingle roof and 15% for a ground mount rack.
60 Cell Panels:
(Both of these options are all-black panels that were designed for excellent appearance on a roof)
Grade A Waaree 315W Mono, 64.6" x 39.0" x 1.6", 40.8 lbs. 25 year manufacturer's warranty. 26 panels/pallet. $0.42/Watt or $132.20/panel.
Grade B Heliene 300W Mono, 65.5" x 39.4" x 1.6", 48.5 lbs. Made in USA. 5 year manufacturer's warranty. 26 panels/pallet. $0.37/watt or $111.00/panel. "Grade B" means cosmetic, non-electrical blemishes.
As I mentioned to you, we have installed about 150 solar electric systems representing over 3 Megawatts of power generation. Panels fail at the rate of 1 in 10,000, and generally the panel will either be dead on arrival or it will go the distance (panel production does degrade at about 0.5%/year). Statistically, very few panel claims are ever made, and even fewer are actually paid, so Grade B panels represent a good investment. Heliene is an excellent Canadian-based company with panel production in Minnesota. The panels we are offering were designed for 25 year warranty but have cosmetic blemishes. Signature Solar normally offers a 1 year "no-lemon" warranty on all of the panels we sell, if the manufacturer does not offer a warranty. This is the first time I have heard of a manufacturer offering a warranty on Grade B panels.