Estimating the expected output of a PV electric system is an involved process. First, there are site-specific variables — like the pitch and orientation of the array, shading, mean temperature and solar irradiance. Then, there are system-specific variables: system size, cell efficiency and degradation rates, module mismatch, panel soiling (the accumulation of dust and dirt), electrical loss due to wiring, etc. Assessed with prudence, these variables can provide a pretty accurate look at the future electricity output (in kWh) of a given system.

Of course, these kinds of calculations are a breeze for solar installation professionals. And any decent quote will present a fairly robust analysis of the expected performance (in kWh) of your system. But what about for us, the laymen, who want a quick and dirty estimate of how much power to expect from a particular panel array?

Well, in simplest terms, the amount of electricity produced by a solar PV system is equal to a fraction of the sunlight that hits the panels. Just like the engine in your car can’t convert all the energy in a tank of gasoline into kinetic energy, solar panels can’t convert 100 percent of sunlight into electric energy. So, we take the amount of sunlight (solar irradiance) and multiply it by the system size (also called system capacity), and then multiply that by a loss factor — sometimes called a derate factor. In rough terms, it looks like this:

Solar irradiance (kWh/square meter/per day)  x  system size (STC kilowatts)  x  derate factor

Where can we find the three components of this expression?

• Solar radiation data for the U.S. are available from the Solar Radiation Data Manual (also called the “Red Book”), published by National Renewable Energy Laboratory. (Note that the amount of radiation in kWh/square meter/day varies considerably depending on the pitch and orientation of the array.)
• Published by the California Energy Commission, the Guide to PV System Design and Installation (PDF) provides a good primer on how to determin an appropriate derate factor, particular for those readers in California.
• Finally, the STC wattage is listed on the back of each panel in your system. Simply multiply the watts by the number of panels in your array and you’ll get your overall system size.

Since we’re taking in VERY general terms here, we’ll take a VERY general example: a 4.5-kW system in Denver, Colorado. (While systems that track the sun are more efficient, they’re also a bit more expensive. For now, we’ll just consider a fixed, flat-panel array.) Here are the variables:

• At an angle of 40 degrees, south-facing panels will receive an average of
  5.5 kWh/square meter/day over the course of a given year
• We’ll take a middle-of-the-road derate factor of 0.70
• Our STC wattage is as listed on the back of our panels: 25, 180-watt (STC) panels = 4.5 kW

Now, it’s just a matter of simple multiplication:

5.5 kWh/square meter/day x 4.5 kW x 0.70 = 17.3 kWh/day = 520 kWh/month = 6,324 kWh/year

To reiterate, this is an oversimplification of an admittedly complicated process — and it relies on averages and estimates. As such, it should only be used as a rough point of reference. For help with sizing your system, feel free to contact us or one of our qualified solar professionals serving your area. As always, feel free to post questions or comments for follow up.