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In several studies of energy payback for amorphous solar cells, with the average solar energy (which varies over Europe between 1,100 and 1,900 kwh/m2/year) landing on a solar panel estimated conservatively at 1,000 kwh/m2/year ("insolation") and an energy efficiency ranging from 5% to 8% (yielding a "harvest" of 50 to 80 kwh/m2/year), energy paybacks of 1.3 to 3 years have been estimated.
These estimates are based upon an Equivalent Primary Energy (EPE) factor of 2.5:1 (that is to say, electrical energy from photovoltaics is compared with electricity derived at an efficiency of 40% from primary energy sources such as fossil fuels). The calculations include a factor to account for degradation of panel efficiency, and also include the energy required to make all of the various materials used in manufacture of the panels (principally glass, plastic, and Silane), [References: Srinivas, 1991; Shah, 1992].
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