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EarthEnergy Blog: Information to help "make your home wherever you go."

Calculate Solar System Output

When you invest in a solar power system, it’s important to take into consideration how much sun your location gets, on average. With this information, you can then calculate how many photovoltaic panels you’ll need to install to produce the amount of power you need. The National Renewable Energy Laboratory (NREL) has two options for determining your solar system’s potential power output.

The PVWatts Calculator will automatically determine your expected kilowatt hours per year, average solar radiation per month, AC energy output (in kWh), and the approximate energy costs saved, when you input your installation address and the following system information (see image):

NREL PVWatts Calculator

With this system information, your solar system output will be automatically calculated. (Click to enlarge)

NREL also has online PDF charts available that show you the average sun hours per day in your area, either by month or as an annual average:

  1. Visit the NREL Redbook website.
  2. Select your state from the list on the first page.
  3. Scroll through the PDF for that state, to find the page for the city closest to your location.
  4. The city page shows the latitude of the location, and gives you a range of options for angles to install your panels. As you look across the chart, you’ll see the average daily sun hours for each month, for each installation angle, with the annual average hours per day at the far right.
    1. Olympia, WA average sun hours by month

      Click to enlarge chart, and follow along with our solar system output calculations

      With the example chart for Olympia, WA (shown above), we’re going to calculate average output for a 100 watt solar panel installed at a 50º angle, facing true south (not magnetic south). Since the latitude for that location is ~47ºN, we’re going to use the “Latitude” tilt line of the chart (because it is closest to our 50º installation angle). The average daily sun hours/day for the year (at the far right on the chart) is 3.6 and we multiply that by our panel wattage (100w), for 360 average watts per day. Most solar systems have a power loss factor of 20% (due to line loss, and other unavoidable inefficiencies), so we multiply our 360 watts by .8, giving us an average daily output of 288 watts for the year. For more exact averages, you can also do these calculations with the monthly average figures in the chart.

      Once you have an idea how many sun hours your solar power system will be getting, you can plan how many PV panels to buy, and whether you should consider wind or hydro power as a secondary renewable energy source for the months when sunny days are shorter and scarcer.