# How do I calculate the real-world output of my solar system?

I am planning a solar array for my house, is there are rule of thumb to calculate the actual out-put of my new system?

Here is a simple formula that delivers a fairly accurate, although slightly conservative yearly average output: (PV array wattage) x (average hours of sun) x 75% = daily watt-hours PV array wattage is the STC panel rating times the number of modules (If you have 10 Kyocera 215W modules the PV array wattage is 2,150W) Average house of sun is the yearly average of noontime-equivalent hours. This rages from 4.0 to 5.5 hours for most of the US. NREL did a great study for this that you can read to calculate average hours: http://rredc.nrel.gov/solar/pubs/redbook The fudge factor of 75 percent takes into account all the real-world effects of dirty modules, dirty air, high humidity, hot modules, wiring losses, small bits of shading, inverter inefficiency and all of the other little things that effect efficiency. For off-grid systems assume 70%. If your array is perfectly shading-free and you are in a dry, high altitude you can use 80%. Thus 2,150 watts x 5.5 hours x .75 = 8,868 watt-hours or 8.86 kWh per day. Remember this is a yearly average. You will see nearly 2x in the summer as in the winter. This is a conservative way to calculate the real-world output, but it is a good rule of thumb.
Submitted
9 years 11 months ago
Jon Steel
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