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 shape, and more goes into that decision.
Your best bet is to get a quote from one of our expert installer partners in your area.
Determining your ideal solar system size
The main variables that go into determining how many solar panels you need for your home are:
- The state where you live
- How much electricity you use
- How much roof space you have
Here’s why these variables matter: Many states and localities have rules that base the maximum allowed system size on 100% or 120% of your average annual usage. On top of that, some states don’t have net metering; that is, they don’t allow you to sell excess power back to the grid.
The final variable is roof space, and it’s the easiest to calculate. If you have a nice, unshaded, south- or southwest-facing roof to work with, the only considerations are the size and efficiency of the solar panels size and the available square footage.
Read on if you’d like to find out how many of those beautiful panels you can cram on your roof, or try our handy solar panel calculator!
Average size and output of a solar panel
There are many many different kinds of solar 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 simpler to see how many panels you can fit and how much electricity the system would be rated for, except for one thing: solar panel setback.
Solar panel setback
When it comes to solar panels, the term setback means a certain amount of space that needs to be clear between the panels and the sides and ridgeline of the roof.
In California, the rule states that a setback of 3 feet is necessary to allow firefighters to access the parts of the roof that are essential to doing their jobs properly. As you can imagine, this eliminates a good deal of available roof space.
Here’s how setback looks in practice:
That 3-foot space all around the edges of the roof (except the bottom) can make a huge difference. We assume it’ll remove about 25% of total roof space from consideration, leaving 75% of your south- or west-facing roof available for solar panels.
Calculating how many solar panels will fit on your roof
The first thing to account for here is that solar panels can only go on a roof that faces south-ish. That could be southwest or southeast, but due south is best. So when calculating your roof’s area, only consider those portions that face south.
Once you know the area of your south-facing roof, follow these steps to see how many panels will fit:
- Take your available square footage and multiply by .75 to account for setback
- Let’s say you have 420 square feet of sun-facing roof: 420 *.75 = 315 square feet available for solar panels.
- Now take your 315 square feet and divide by 17.5: 315 / 17.5 = 18 panels would fit on your roof.
So with 420 square feet of roof space, you’d get a 4.77-kW system made up of 18 panels. You can do the calculation yourself or use our handy little solar calculator below to find out!
How much energy your solar panels will 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 4.77 from above) by the number of the map color in your area. For us it’s: 4.77 x 1,400 = 6,678.
- 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: 6,678 x 0.78 = 5,209 kWh generated per year.
How much money solar panels can save you
Now this 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 5,209 kWh/year = $573 saved in one year with our 4.77-kW solar panel system!
Now that you’ve read about all that math, let us help you do it: