PV energy payback time

Producing electricity with photovoltaics (PV) emits no pollution, produces no greenhouse gases, and uses no finite fossil-fuel resources.
The environmental benefits of PV are great. But just as we say that it takes money to make money, it also takes energy to save energy. The term “energy payback” captures this idea.
How long does a PV system have to operate to recover the energy—and associated generation of pollution and CO2—that went into making the system, in the first place?

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http://www.nrel.gov/docs/fy05osti/37322.pdf