How to Calculate How Much Solar Panels Do I Need?

Calculating how many solar panels you need is fairly easy if you have the relevant information. It all begins with the load you need solar to supply. If it’s a home, how many kWh does it use?

Use the Following Steps for Solar Panel Sizing:

What is the load? How many kWh does your home use?
What is the irradiance in your location (Peak.sun-hours)
Use the load and irradiance to find theoretical solar system size
Adjust solar size to account for system losses
Divide adjusted solar system size by individual panel watts

Calculate the number of solar panels your home needs by estimating the previous year’s energy bill and using your location’s irradiance value in kWh/m2/year (Peak-sun-hours) to calculate the theoretical solar production needed. Adjust theoretical solar kWh by the loss factor of 1.44 and divide by the individual solar panel watts rating.

How Many Solar Panels Are Needed to Power an Average House?

The average house size in the US is 2500 square feet, which should logically equate to the average energy usage of 11000kWh per year, or 30kWh per day.

However, there are variations in house size from state to state and also in solar panel power output. This is because the sun’s energy, or irradiance, varies by geographic location.

Table – Average House Size in Square Feet – 10 US States Compared

U.S. State

Area (Square Feet)











New Mexico








Historical values for irradiance can be found for your location. The average irradiance value for the USA is 5.322 kWh/m2/day, so I’ll use that value of peak-sun-hours for our calculation.

Using average US values, you can find the theroretical solar power output by dividing the daily kWh by the irradiance value in daily peak-sun-hours:

Solar power required = 30000 watt-hours/5.3 peak-sun-hours = 5660 watts

If 300 watt solar panels were used, the number of solar panels for the average US house would be:

5660 watts / 300 watts = 18.86 (19) solar panels

However, all solar PV systems have losses of about 23%. This can be taken into account my multiplying the solar power required by 1.4:

Adjusted solar output = 5660 x 1.4 = 7924 watts

Using 300 watt solar panels, the actual number of solar panels needed would be:

7924 watts / 300 watts = 26 solar panels

How Many Solar Panels Do I Need for a 3 Bedroom House?

The average size of a US home with 3 bedrooms and 2 bathrooms is 1300 square feet. I previously worked out that solar requirements with average irradiance of 5.3 peak sun hours per day is 3.17 watts per square foot.

Cost of Solar Panels for 3 Bedroom House:

1300 square foot x 3.17 = 4121 solar watts

If 300 watt solar panels are installed, the number of panels required is found by:

4121 watts / 300 watts = 14 solar panels

At just under $3/watt average installation cost, a 4000 watt solar solar panels system would cost $12000 excluding any incentives.

How Many Solar Panels Do I Need to Power a 4 Bed House?

The average size of a US home with 4 bedrooms and 2 bathrooms is 1700 square feet. In the last section I worked out that solar output needed with average irradiance of 5.3 peak sun hours per day is 3.17 watts per square foot.

Solar calculation for 4 bed home:

1700 square foot x 3.17 = 5389 solar output in watts

If 300 watt solar panels are used, then the number of panels needed is found by:

5389 watts / 300 watts = 18 solar panels

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