Energy Payback Time (EPT) = Eo/Eg
Eo: Energy consumed to manufacture photovoltaic systems
Eg: Annual energy output by photovoltaic systems

Module Efficiency
Annual Production Scale (MW)	10	30	100
a-Si	8.0%	10.0%	12.0%
poly-Si	11.9%	12.3%	13.2%

Source: PVTEC's "Investigation to Evaluate Photovoltaic Generation"
Report on the investigation commissioned by the New Energy and Industrail Technology Development Organization (NEDO) of Japan during FY1996