How to Calculate How Many kWh Your Solar System Produces

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The Earth receives over 84 terrawatts of solar power every day.

Photovoltaic, or solar power, systems convert light directly into electricity, according to the National Renewable Energy website. Electrical output varies and is dependent upon factors such as the amount of available light, the position and angle of the solar panels, ambient temperature, the efficiency of the panels and the voltage supplied by the system. Calculating how many kilowatt hours of electricity are produced by a solar system is often a 'best guess' activity because conditions can change in seconds, but a good estimate of the average power production can be made using straightforward techniques.


  1. Solar Systems Connected to Your Utility Company

    • 1

      Get your utility company to install a net meter if your solar system sends surplus power back to your utility company. This constantly monitors the total output of your solar system and displays it in kilowatt hours.

    • 2

      Note the meter reading at the start of the period over which you want to monitor electrical output and note the reading again at the end of the period.

    • 3

      Deduct the first reading from the second reading. The result is the amount of electricity produced by your system over the given period. For example, if the original reading was 200 and the final reading was 600, your system produced 400 kWh of power: 600 - 200 = 400.

    Stand Alone Solar Systems

    • 4

      Establish the "peak current rating" for your panel. This is given in the manufacturer's technical data and is also printed on the panel. The value is given in amps.

    • 5

      Determine the "number of peak sun hours" to which your system has been exposed over the period for which you're calculating output. Tables and maps detailing peak sun hours per day can be obtained from university outreach departments, local authorities and government agencies such as the U.S. Department of Energy. Multiply the hours per day by the number of days. For example, if your system is exposed to four peak sun hours per day for 10 days, it's been exposed to 40 peak sun hours: 4 x 10 = 40.

    • 6

      Multiply the peak current rating by the number of peak sun hours to obtain the amp hours, or Ah, produced by your system. For example, with a peak current rating of 6 amps and 200 peak sun hours, the amp hour value is 1,200 Ah: 6 x 200 = 1,200.

    • 7

      Multiply the amp hour value by the output voltage, then divide the result by 1,000 to determine the kilowatt hours. For example, 1,200 Ah at a voltage of 12 volts gives 14.4 kWh of power: 1,200 x 12 = 14,400. Divided by 1,000, that equals 14.4 kWh.

Tips & Warnings

  • The conditions under which power is generated can vary greatly, so you can't calculate a "one off" power output that's accurate all the time.

  • Peak sun hours are found by taking the entire solar irradiance that falls on one square meter during the day and dividing it by 1 kW. For example, if a location receives four peak sun hours, it has received the equivalent of four hours at 1 kW per hour, although it may have been accumulated at varying rates over a much longer period.

  • Keeping panels clean, in full sunshine and pointing directly toward the sun will maximize output.

  • Manufacturer's claims for the output of their panels may exceed your own experiences. The maximum power output is determined under ideal conditions and is unlikely to be replicated at home.

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  • Photo Credit Comstock/Comstock/Getty Images


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