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We get asked this question frequently and with such a simple question, one would suspect a relatively easy answer. However, just as the cost of a solar system has many variables, the number of solar panels needed for your business or home also varies. Here is a rough explanation on how to figure out how many solar panels you may need.

Sun Hours

Depending on where you are in the country, each location has a different amount of peak solar production hours each day. We call this measurement ‘solar radiation hours’ or ‘sun hours’ and they typically range from 3.5 to 5 hours per day. For example, the Southwest of the United States has more sun hours than the Northeast, due to geographic location. More sun hours per day = less solar panels needed.


What is Your Electric Usage

We also need to know how many kWh (kilowatt-hours) you use each year (this is why we request your electric bill during the solar assessment). Using this information, we are able to take an average kWh usage per month and size your solar system accordingly. For example, if you use 12,960 kWh per year (1,080 kWh/month and 33 kWh per/day) and are located in a location with 5 hours of sunlight per day you can make the following calculation:

AC Power Needed=Average kWh per month/30 days/Average Sun Hours Per Day

7.2 kW AC=1,080 kWh/30 days/4 sun hours per day

Because solar panels produce DC power, we need to convert from AC to DC power to find out how many panels will be needed. We calculate DC power by using the following calculation:

7.2 kW AC/.8= 9 kW DC



Panel Size

Solar panels come in different sizes with the current average size panel being around 300 W DC. Currently, the wattage of modules that we use ranges from 260-335. A larger solar panel size in regards to wattage equals less solar panels needed. Continuing on with the same example, we now need to convert the 9 kW solar system size to watts to find out how many solar panels are needed. Here is that calculation:

9 kW x 1,000 (1 kilowatt=1,000 watts)= 9,000 watts (W)
9,000 W/300 W solar panel=30 solar panels needed.


Space Issues?

All of the calculations so far have assumed that the solar system would cover 100% of total electric usage. However, in some cases, it may not be possible to cover 100% of total usage if there isn’t enough physical space to install a solar system. Solar panels typically measure around 3’3” x 5’6”, which occupies 18 sq/ft of roof space, or 3’3” x 6’6”, which occupies 21 sq/ft of roof space (learn more about solar panels styles here). In our example, if we needed thirty 300 W solar panels, and they each took up approximately 21 sq/ft of roof space, we would need approximately 630 sq/ft of roof space to install the whole solar system.

21 sq/ft coverage x 30 solar panels=630 sq/ft of roof space

If there isn’t 630 square feet of roof space available, the solar system may need to be downsized or installed on the ground in order to cover the total electric usage.




While this article provides a basic overview, it does give a quick, rough way to calculate the approximate number of solar panels required to cover your electric bill. Our department of highly trained estimators take many other factors into account, including shade, temperature, and inverter efficiency to get a much more accurate production number. Contact us to find out your solar system size!


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