http://en.wikipedia.org/wiki/Solar_cells#Solar_cells_and_energy_payback

Solar cells and energy payback
Further information: Low-cost photovoltaic cell

In the 1990s, when silicon cells were twice as thick, efficiencies were much lower than today and lifetimes were shorter, it may well have cost more energy to make a cell than it could generate in a lifetime. In the meantime, the technology has progressed significantly, and the energy payback time, defined as the recovery time required for generating the energy spent for manufacturing of the respective technical energy systems, of a modern photovoltaic module is typically from 1 to 4 years[25][26] depending on the module type and location. Generally, thin-film technologies - despite having comparatively low conversion efficiencies - achieve significantly shorter energy payback times than conventional systems (often < 1 year).[27] With a typical lifetime of 20 to 30 years, this means that modern solar cells are net energy producers, i.e. they generate significantly more energy over their lifetime than the energy expended in producing them.[25][28][29]
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