Energy Return on Investment (EROI) is the ratio between the amount of energy expended to obtain a particular energy resource and the amount of usable energy acquired from that resource. When the EROEI of a resource is equal to or lower than 1, that energy source becomes an “energy sink”, and can no longer be used as a primary source of energy.

http://en.wikipedia.org/wiki/EROI

A related concept is the energy pay-back time, i.e. the time required to produce an amount of energy as great as what was consumed during production.

In photovoltaics, crystalline silicon PV systems give energy payback times of 1.5-3.5 years, depending on the installation location. Expected efficiency improvements in silicon solar production techniques will reduce the required energy inputs, thereby bringing an energy payback of 1 year or less for silicon-based PV.