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How Many Solar Panels Do I Need?

Man installing solar panel on roof of home

The average American home needs between 15 and 19 solar panels. Installing that many solar panels would cost between $12,000 and $17,000 after the federal solar tax credit.

Your energy usage may not line up exactly with the U.S. average, so here's a way to calculate your home's requirements. The number of solar panels you need depends on several other factors as well, like the location of your home and what kind of solar panels you choose. Let's dive in.

How to calculate the number of solar panels you need

If you want to put pen to paper and work out the calculations for yourself, follow these steps to find how many solar panels you need. If you want to use an online calculator you can find several options through Google. The information below, however, will help you determine if the calculator is accurate based on your home's specifications.

Step 1: Determine your energy usage

Locate your most recent power bill to see your monthly electricity consumption. The total amount of electricity used will usually be shown at the bottom of the bill in kilowatt-hours (kWh).

Your yearly energy usage will give you the best estimate as energy usage fluctuates in different seasons. To find this figure, use your monthly electric bills for the past year, add the electric usage together, and divide by 12.

Step 2: Find how much energy solar panels produce in your area

Next, you need to find the amount of sunlight your area receives. This is usually measured in something called "peak sun hours", which is essentially the intensity of sunlight in your area. A peak sun hour is the equivalent of 1000 W/m² of sunlight for an hour.

Virginia average 3.5 - 4 peak sun hours a day. To calculate your monthly peak sun hours (which is needed to determine your solar needs), you multiply 3.5 - 4 by 30 to get Virginia's average of between 105 and 120. On average, the U.S. receives between 120 and 150 peak sun hours per month, so Virginia is below average for sunshine.


In the United States, any place that gets 4 peak sun hours or more is considered a good

location to produce useful amounts of solar energy.


The monthly peak sun hours total determines how many kilowatt-hours of electricity 1 kilowatt (kW) of solar panels will produce in one month. So, Virginians can expect between 120 and 150 kWh of electricity per month.

Step 3: Calculate the solar system size you need

Since you now know how much 1 kW of solar will produce in your area, you can determine what size solar system you need to cover your electricity needs.

Divide your monthly electricity usage by 120 to 150 (you can estimate your sun exposure taking into account topography, which direction your roof faces - south is best, and shade factors like trees) to find the system size you need in kW. For the average home in the Virginia the calculation would look something like this:

893 kWh consumed / 125 monthly peak sun hours = 7.14 kW of solar

Step 4: Figure out how many solar panels you need

To figure out how many solar panels you’ll need, take the solar system size and multiply it by 1,000 to convert the size from kilowatts to watts. Using the example above, it looks like this:

7.14 kW of solar x 1,000 = 7,140 watts of solar

Take the size of the system in watts and divide it by the wattage of the solar panels you want to install. The average solar panel in the U.S. is about 370 watts in size. For the sake of our example, that means you would need 18 solar panels to cover your energy usage.

7,140 watts of solar / 370-watt solar panel = 19 solar panels

Step 5: Determine how much roof space is needed for solar panels

The average solar installation will require between 260 and 340 square feet of roof space. To find out how much roof space your solar system needs, multiply the number of panels you need by 17.55 square feet, which is the area of most residential solar panels sold today.

19 solar panels x 17.55 square feet = 333.45 square feet of roof space

If you have limited roof space, you may need to get high-efficiency panels, or panels with a higher power output so you can install fewer panels while still covering your energy needs.


Sources: Solar Reviews,


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