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.
Solar Systems Connected to Your Utility Company
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.
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.
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
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.
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.
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.
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.
- University of Oregon -- The Electronic Universe: Basics of Solar Energy
- National Renewable Energy Laboratory: Solar Photovoltaic Technology
- Wind & Sun: Calculating the Output of Photovoltaic Modules
- California Energy Commission: Buying a Photovoltaic Solar Electric System - A Consumer Guide
- U.S. Department Of Energy: Clean Energy Choices
- Sandia National Laboratories: Glossary - Peak Sun Hours
- Photo Credit Comstock/Comstock/Getty Images