One of the most-asked questions in the solar installation business is “how many panels do I need?” And the answer, as always, is “it depends.” There are many variables that go into deciding the size of system you need; cost, the amount of sun you get, the amount of electricity you want to generate, roof size, and more goes into that decision. Your best bet is to get a quote from one of our expert installer partners in your area.
People also ask a related question: “how many solar panels can I fit on my roof?” The answer is, again, “it depends.” But it depends on a whole lot less! If you have a nice, unshaded, south- or southwest-facing roof to work with, the only variables are panel size and available square footage.
Read on if you’d like to find out how many of those beautiful panels you can cram up there, or skip ahead to our handy calculator!
There are many many different kinds of panels out there in the world, and they vary in size, efficiency, and energy produced. Luckily, the sunny state of California keeps excellent records on all the solar installations it approves, so with some quick pivot table work, we discovered the top 5 solar panels on the market (representing 30% of all installations). Then we figured out the average size of those panels and how much energy they are rated to produce. Here’s a look at what we found, with the averages at the bottom:
|MAKE/MODEL||WIDTH (IN.)||LENGTH (IN.)||WATTS||WATTS/SQ. FT.|
SW280 Mono Black
So how many solar panels will fit on my roof?
As you can see above, the average solar panel these days is about 3.25 feet by 5.4 feet (about 17.5 square feet) and puts out about 265 watts of electricity. That makes it simple to see how many panels you can fit and how much electricity they would be rated for:
- Take your available square footage and divide by 17.5.
Let’s say you have 385 square feet of sun-facing roof: 385 / 17.5 = 22 panels would fit on your roof.
- Each panel produces 265 watts, but systems are measured in kilowatts (kW), so you have to divide by 1,000: 265 / 1,000 = .265 kW per panel.
- Multiply those 22 panels by .265 kilowatts each: 22 x .265 = 5.83 kW.
So with 385 square feet of roof space, you’d get a 5.83-kW system made up of 22 panels. You can do the calculation yourself or use our handy little solar calculator below to find out!
And how much energy will they produce?
The big factor in finding out how much juice you can squeeze outta your panels is where you’re live, and thankfully, the folks over at the National Renewable Energy Laboratory have made the handy map below to make it easy. Check it out:
- Find your city in the map and match it to the numbered area.
Let’s start with our home, Portland, Oregon. Our city lies within the 1,400 band, so 1,400 is our number.
- Multiply the number of kW your system is rated for (we get 5.83 from above) by the number of the map color in your area. For us it’s: 5.83 x 1,400 = 8,162.
- Next, multiply that number by 0.78. Why 0.78? It represents the percentage of electricity you can expect to capture, based on inverter efficiency, panel performance, and losses from wiring. We calculate: 8,162 x 0.78 = 6,366 kWh generated per year.
OK, but what I really want to know is: how much money can I save on electricity?
Now that one really depends. The biggest two factors that determine how much money your solar panels save you are incentives and electricity cost. Incentives are tricky, because they vary from state to state (and sometimes from month to month). But electricity cost is easier to figure out, because you get a bill every month with the cost per kilowatt-hour (kWh) right on there!
Now that you know how much electricity your panels can produce in a year, look at your utility company’s website or check your bill for your rate. Be sure to add in the per-kWh charges for transmission and distribution, because you’ll be saving on those too!
Last of all: multiply your electric rate by the number of yearly kWh from the last step. In Portland, we pay about $0.11/kWh for electricity. Here’s how that looks for us: $0.11/kWh x 6,366 kWh/year = $700.29 saved in one year with our 5.83-kW solar panel system!
Now that you’ve read about all that math, let us help you do it: