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Question DetailsAsked on 7/1/2014

Solar Energy calculations

If I purchase a package of 8 275 watt panels, how many KWH can be expected, flat roof,southern exposure, no obstructions?

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1 Answer


Depends on efficiency of panels (275W is nominal, not true output), what part of the country you are in (sunlight percentage, latitude) - manufacturer website should have a computation page. If not, Department of Energy - Energy Star website does (once you get true rating on panel), as does UC Berkeley, among others.

Don't forget to include line and system losses, conversion losses if converting to household energy (as opposed to direct resistance heating or hot water heating) - and remember to check typical info on DOE website on loss of efficiency due to dirt and such - can rapidly take off 20-30% of your power, and excessive cleaning damages the panels, so typically (especially in dusty areas and dirty cities) your average power production can go way down versus expected.

Also - I would STRONGLY advise you spend a LOT of time thinking how these are going to be mounted - because if you don't want them landing in Florida in the first big wind, they HAVE to be tied down substantially, which means roof penetrations - NOT a good idea if avoidable on flat roofs (or any roof, for that matter). Also, flat roof install, unless extremely rigidly fastened, will not be able to provide the correct exposure angle for maximum energy - flat roofs need a near vertical install for high energy output except in the southern-most part of the country.

And pay attention to electrical system modification and intertie costs, permitting issues, possible roof structure reinforcing needs, etc.

Answered 6 years ago by LCD

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