How much solar power can my roof generate?

In a perfect world, the average roof in the U.S. can generate around 21,840 kilowatt-hours (kWh) of solar electricity annually—that's more than most homes need. But also, the world isn't perfect.

Realistically, your roof’s solar generation potential will be less than that. It’ll likely still exceed your typical household energy needs, but real-world constraints like roof space, sunlight exposure, and equipment specifications play a huge role in your panels’ actual generation. 

With so many variables at play, it can take time to understand what kind of solar panel system to install at your home. Let’s walk through how to calculate the amount of solar power your roof can generate based on its size, orientation, and angle—as well as the solar panels you install.

In some cases, way more than you probably need. According to our calculations, the average-sized roof can produce about 21,840 kilowatt-hours (kWh) of solar electricity annually—about double the average U.S. home's usage of 10,791 kWh.

But remember, we’re running these numbers based on a perfect, south-facing roof with all open space—which won't be the case 99% of the time. Roofs have chimneys, vents, rafters, and more that reduce the overall space available for solar panels. Plus, most installers won't install panels right up to the edge of your roof, which reduces the open space even more. 

We’re here to help you understand how to calculate your solar generation potential, but you should work with your installer to figure out your home’s individual energy needs and capabilities. 

Calculating solar generation potential

We use the following assumptions to calculate solar generation potential in an ideal scenario:  

• 850 square feet of usable roof space for solar: The average U.S. roof is about 1,700 square feet. You should never put panels on northern roof planes. So with a north/south roof, that gives you 850 square feet.

• 400-watt solar panels that are 20 square feet in size: This is the most frequently quoted panel power output on EnergySage.

• 1.3 production ratio: This is the U.S. median production ratio, which is the estimated energy output of a solar panel system relative to its actual size in watts (W).

First, determine how many solar panels you can fit on your roof. Assuming all of the roof space you've got is usable for solar (which, again, usually isn't the case), that's 42 panels (850 square feet divided by 20 square feet per panel). 

Multiplying the number of panels by the 400-watt power output of each panel gets us a system size of about 16.8 kW. 

Finally, 16.8 kW translates to roughly 21,840 kWh of production per year when you factor in the production ratio (16,800 W x 1.3).

Solar panels don't come in one standard size. But most of the popular home panels today are about 20 square feet. To calculate how many panels can fit on your roof, divide your open roof space by 20 square feet (or however large your particular solar panels are). For example, if you have 500 square feet of open, available roof space, that's enough space for about 25 solar panels.

One of the significant difference-makers of these calculations is geographic location, which directly impacts the hours of quality sunlight your solar panel system will get, and, therefore its production ratio. 

We used 1.3 for our production ratio in the example above because it's the median we see of all the states on EnergySage. Here's how the average production ratio, and our estimated solar energy production, change by state:

How much solar energy can you generate on your roof by state?

*Assumptions: 850 square foot roof, 20-square foot/400-watt solar panels

Another way to segment solar generation potential is by roof size. Below is a chart comparing solar generation potential based on roof size, assuming all of the same metrics as before: 400-watt solar panels, 20-square-foot panels, and using every inch of roof space available for solar. 

How much energy can differently-sized roofs produce?

*Assumptions: 20-square-foot/400-watt solar panels, 1.3 production ratio

Of course, it's never quite that simple. A few factors to consider that’ll adjust your personal solar generation potential: roof space, location, and equipment specs.

Roof space

The more usable your space is, the more solar panels you can feasibly add to your system. More panels equals more energy production, so a larger roof means more capacity to generate solar electricity.

Location/amount of sun

The amount of sunlight that actually hits your solar panels is a key factor when calculating how much solar energy your roof can generate. You can put all the solar panels you want on your roof, but at the end of the day, they’ll only produce electricity when the sun shines. 

The amount of energy your system produces relative to its actual rated size is known as the production ratio. A solar panel system's production ratio is the ratio of the estimated energy output of a system over time (in kWh) to the system size (in W). 

These numbers are rarely 1:1. Your production ratio will change depending on how much sunlight your system gets (primarily based on your geographic location but also influenced by roof angle and directional orientation). In the U.S., production ratios are usually between 0.9 and 1.6.

For example, a 10 kW system that produces 13 kWh of electricity annually has a production ratio of 1.3 (13/10 = 1.3).

Roof orientation and tilt

Ideally, your solar panels will be installed on a south-facing roof at an angle of about 30°. These are the optimal conditions for solar panel production. The closer you get to this, the more electricity your panels produce. 

Equipment size, performance, and power

Solar panels with a larger power-to-size ratio will produce more electricity per square foot. As panel technology continues to improve, the amount of space needed to produce enough energy for your home will decrease. Just keep in mind that panels with higher efficiency and wattage usually come at a higher cost, too. 

At the end of the day, the easiest way to accurately determine how much solar power your roof can generate is to talk with installers. They design solar panel systems every day and will be able to assess your roof's unique features and provide you with a production estimate.