http://en.wikipedia.org/wiki/Photovoltaics#Energy_payback_time_and_energy_returned_on_energy_invested

Energy payback time and energy returned on energy invested

The energy payback time is the time required to produce an amount of energy as great as what was consumed during production. The energy payback time is determined from a life cycle analysis of energy. The energy needed to produce solar panels will be paid back in the first few years of use.[73]

Another key indicator of environmental performance, tightly related to the energy payback time, is the ratio of electricity generated divided by the energy required to build and maintain the equipment. This ratio is called the energy returned on energy invested (EROEI). Of course, little is gained if it takes as much energy to produce the modules as they produce in their lifetimes. This should not be confused with the economic return on investment, which varies according to local energy prices, subsidies available and metering techniques.

Life-cycle analyses show that the energy intensity of typical solar photovoltaic technologies is rapidly evolving. In 2000 the energy payback time was estimated as 8 to 11 years[74], but more recent studies suggest that technological progress has reduced this to 1.5 to 3.5 years for crystalline silicon PV systems [70].

Thin film technologies now have energy pay-back times in the range of 1-1.5 years (S.Europe).[70] With lifetimes of such systems of at least 30 years[citation needed], the EROEI is in the range of 10 to 30. They thus generate enough energy over their lifetimes to reproduce themselves many times (6-31 reproductions, the EROEI is a bit lower) depending on what type of material, balance of system (or BOS), and the geographic location of the system.[75]