Calculations for Solar Electric Article In cloudy western Washington a local installer of PV solar is promoting a three-panel system for an expected installed cost of approximately $5,000. Solar radiation on a surface perpendicular to the sun is approximately 1000 watts per square meter or 93 watts per square foot. A typical solar panel (Siemens #SP75) is 6.8 SF in area and can produce 75 watts of power in full sun. It is producing power at the rate of 11 watts/ SF (75 watts / 6.8 SF). This works out to be a maximum efficiency of about 12% (11 watts / 93 watts = .118 or 12%). Since 10% to 30% is lost in conversion to AC and battery storage, a good working conversion efficiency might be 10% of available sunlight. Seattle weather stations have kept daily and monthly solar insolation (sunlight) records for decades. When measured on a horizontal surface and averaged throughout the year, the total available solar energy in this climate averages 296 watt-hrs/ SF/ day*. Of course, there is more in the summer, less in the winter and many daily variations. For a PV panel, let's assume that with tracking and proper orientation we can add 50% to this usable horizontal energy. That would give us 444 w-h/ SF/ day. At 10 % efficiency, the Siemens solar panel can convert our sunlight to 44 w-h/ SF / day or 300 w-h/ day per panel (6.8 SF x 44 w-h/ SF /day = 299.2 w-h). A three panel system can be expected to average about 900 w-h per day in net energy produced. This is enough power to run a 100 watt light bulb for nine hours. It will also run an 18 watt compact fluorescent bulb (that produces about the same amount of light) for 50 hours. In other words... Spending $5,000 on the PV solar system produces 900 w-h per day in produced power. Changing a 100 watt incandescent light bulb that is used 12 hours a day to an 18 watt compact fluorescent produces 984 w-h of savings. (100 w - 18 w = 82 watts x 12 hrs = 984 w-h) The two alternatives produce approximately the same amount of energy. The cost differences are astounding.
	Back to Solar Electric Article        CountryPlans.com HOME * The Passive Solar Energy Book, by Ed Mazria, p. 356.