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Energy payback time (EPBT) refers to the "time required for any energy producing system or device to produce as much energy as was required in its manufacture." For a large wind turbine, for example, the EPBT is roughly three to eight months. "The life cycle assessment of a 3.0 MW wind turbine indicates that it would have to generate electricity for only 6.8 months, of their assumed 20 year useful life, before it produces as much energy as is used during the manufacturing phase," writes Jenna Watson at TreeHugger.com. This . means the turbine model earns its own worth more than 35 times during its energy production lifetime."
For solar electric panels, the EPBT time is about 16-20 months. Here's how: "Roof-mounted photovoltaic systems have impressively low energy payback times, as documented by recent (2004) engineering studies. The value of EPBT is dependent on three factors: (i) the conversion efficiency of the photovoltaic system; (ii) the amount of illumination (insolation) that the system receives (about 1700 kWh/m2/yr average for southern Europe and about 1800 kWh/m2/yr average for the United States); and (iii) the manufacturing technology that was used to make the photovoltaic (solar) cells."
