If you used The Solar Nerd calculator, you’ll have an estimate of how many solar panels you need to generate all of your electricity. But then the question is: will that many panels actually fit on your roof?
You might know the square footage of your house, but that doesn’t tell you the area of roof that’s available to fit solar panels. What you want is a big rectangular area without any shade. A south or west facing roof is usually best, but east can be viable in some cases.
But not all rooftops are simple rectangles. Hip, gabel, and mansard roofs, for example, are designs common on American homes, and they each have a different geometry that changes the amount of usable area. So do features like dormers, ventilation pipes, and chimneys.
Another possible situation is that you have a good amount of roof area, but it’s shaded by trees or neighboring buildings.
Finally, you may have local fire setback codes that require you to leave a free pathway on the edge of your roof for access to firefighters.
All these considerations will affect what the final layout of your solar panels will be, which is why you want to work with a professional installer to give you a design proposal. But with the method I’ll describe in this article, you can make a decent estimate of your solar panel layout in just a few minutes.
Here’s a secret: most solar installers use a method similar to the one described in this article to prepare system layouts for customer quotes. While they use professional software, solar companies almost always make use of satellite images from Google Earth or other sources as a starting point.
A responsible company will always come to your house to ground-truth the design by verifying measurements and looking for issues that aren’t always visible in satellite photos like vent stacks, shading, and roofing issues. (Never work with an installer company that makes you sign a contract without first visiting your house.)
But pretty much all solar installers start the design process by using satellite images. Here’s how you can do a rough draft yourself.
There are websites that will let you use satellite maps to draw an area on your roof to get the square footage. You can find a few of these on the internet, but one I like is on mapdevelopers.com:
When you go to the area calculator, you’ll see a screen like this:
In the address field, enter your street address, city and state, separated by commas (step 1). Then click the ‘zoom to address’ button, which will place a red pin on your house (step 2).
Then, switch to the satellite view so that you can see your actual house (step 3).
You’ll need to click the + icon on the lower right corner to zoom all the way in (step 4).
After that, click on the map to draw a polygon around the area of the roof where you think your panels should go. You will click on each corner of the polygon, and then click back to the first point to close the polygon (step 5).
If you’ve done it correctly, the calculated area of your polygon will be displayed above the map (step 6).
If you have a proper safety harness, you could go up on the roof and take a direct measurement with a tape measure. That’s risky for personal safety, and walking with a heavy foot on your roof can damage your shingles. So I don’t recommend that.
Instead, if you have a rectangular section of roof, you could take the measurement from the ground. This should be more accurate than using a tool based on satellite photos.
Take your measurements from the corners of the roof where they meet the ground:
Once you have X and Y, multiply them to get the area on the ground. So, if your X = 25 feet and Y = 12 feet, your area on the ground would be 300 feet². But that’s not the area of the roof, because it is sloping. We need to do a little trigonometry.
You need to measure the angle of the roof, and you can do that with a smartphone app like Measure for iOS or Bubble Level for Android. Measure comes pre-installed on iPhones, so you should have it already.
Of course, this isn’t as accurate as going up on a ladder to take a measurement, but it sure is a lot easier, and good enough because we’re just doing this exercise to get a rough estimate of our panel count.
To take a measurement, stand underneath the roof section you want to measure, and hold up the edge of your phone so that it’s parallel with the roof. Here’s what it looks like using Measure:
Let’s round it off and call it 35°. To get the area of the roof, you then need to divide the area you calculated on the ground by the cosine of your angle in degrees. An easy way to do this to enter this equation into Google.
For our example, we’re dividing 300 square feet by the cosine of 35 degrees. Type it into Google like this:
The result will appear in Google with a handy visual calculator:
Our roof area is 366 square feet. You can follow this link for a direct example.
Pretty handy! While this works well for rectangular sections, triangular sections of roof are trickier.
That’s my house in the photos above, by the way. I’m lucky to have a simple roof to work with, but yours might not be a straight rectangle. If you have triangular sections of your roof, you’re probably better off using the online tool rather than trying to estimate it from the ground.
It’s pretty common to use multiple sections of your roof for solar panels.
For example, having some panels facing south and others facing west could be really useful when you have a time-of-use plan with your utility company. Your south panels will capture the most energy in total throughout the day, but your west panels will generate more during the critical peak load time in the early evening when the grid is strained.
In any case, calculating your panel count is still easy. Whether you’re using the online area calculator or measuring from the ground, just measure the roof sections individually and use the calculator at the bottom of this article to see how many panels you can fit on each section.
If you have panels set up in multiple groups (called strings), you will either need an inverter that can support multiple strings, or use microinverters. Microinverters give you maximum flexibility but cost more. To learn more, you can read my article that explains how solar inverters work.
Just be aware of things on your roof where panels can’t go, such as vent stacks, chimneys, and heavily shaded areas. Also, some cities may have local fire setback codes that prevent you from placing panels right up the edge of the roof.
Many state and local jurisdictions have building codes that require solar panel installations to leave an unobstructed pathway around the edge of the rooftop. These are called fire setbacks, and they’re there to give firefighters better access to the roof in an emergency.
These requirements don’t exist everywhere, so check with your local authorities. But they are required in California. The exact size of the fire setback depends on the size of the building, but for an average sized home, each roof section that has solar panels requires two clear 36-inch wide pathways running along the edge of the roof. Here’s what that looks like:
This is an example for a simple roof, but the requirements differ for other types of buildings. To learn about this in detail, you can refer to the webinar and document linked in this article from CALSSA.
Once you have an estimate of your available roof area in square feet, enter that number in the calculator below. It will tell you how many average sized solar panels will fit in the space.
Warning: this calculator doesn’t take into account fire code setbacks, because those vary from jurisdiction to jurisdiction. If you know what your setback codes are, you can simple subtract the area of the required pathway from the total roof square footage.
Of course, the methods described in this article for making a rough calculation are just to give you a quick way to figure out how many panels you might be able to fit on your roof. It’s no substitute for working with a professional solar installer who will include a site layout with your solar quote and explain in detail which sections of your roof are best suited for solar panels.
They’ll also be able to identify obstructions, shading, and other issues.
When you’re ready to work with a professional contractor for your home solar panel installation, fill out our form to get quotes from up to three qualified solar installers.