Energy payback time of photovoltaic systems
The energy payback time is the time needed for the PV system to generate the amount of electric energy that substitutes the amount of primary energy that was needed to produce it.
The energy payback time = Primary energy input / Substituted primary energy related to the electric energy output per year. Note that the lifetime of the PV system is not included.
Energy input is calculated using the “Cumulative Energy Demand” (CED) method which is the total life cycle primary energy consumption.
The energy input of renewable energy resources like wind, solar and geothermal equals the amount of energy harvested (or converted).