Energy payback time
The time an energy system must operate to generate as much energy as was used to manufacture, install and decommission it.
Energy payback time (EPBT) compares the cumulative energy demand of an energy system over its life cycle (mining, manufacturing, transport, installation, maintenance and end-of-life) with the energy it delivers each year. It is usually expressed in primary energy terms, so electricity output is converted using the grid's primary energy factor.
Here ηgrid is the efficiency with which the reference power system converts primary energy to electricity, often about 0.30 to 0.35.
Typical values:
- Crystalline silicon PV: about 1.0 to 1.3 years in central Europe (around 1,000 kWh/kWp per year), and under 1 year in southern Europe, the Middle East and Australia (Fraunhofer ISE Photovoltaics Report). Around 2000, EPBT was 3 to 4 years.
- Thin-film CdTe: under 1 year in many locations.
- Onshore wind turbines: about 5 to 12 months (manufacturer LCAs from Vestas and Siemens Gamesa).
- Offshore wind: about 7 to 12 months.
With lifetimes of 25 to 30 years, PV systems return roughly 20 to 30 times the energy invested. EPBT is closely related to EROI, which divides lifetime output by embodied energy. Neither should be confused with the financial payback period. Results depend on manufacturing location, silicon purification route and grid mix assumptions, harmonised in IEA PVPS Task 12 guidelines.
Related terms
All terms →- EROI (Energy Return on Investment)
The ratio of usable energy delivered by an energy source over its lifetime to the energy invested to obtain and deliver it.
- Lifecycle emissions
Total greenhouse gas emissions of a product or power source across its full life, from raw material extraction and manufacturing through operation to decommissioning.
- Payback period
The time needed for the cumulative savings or revenues of an investment to equal its initial cost.
- Primary energy
Energy in its raw natural form before conversion, such as crude oil, coal, natural gas, uranium, biomass, and the electricity or heat from renewables.
- Specific yield
Annual energy produced per unit of installed PV capacity, expressed in kWh/kWp per year; combines solar resource and system performance.
- Degradation rate
The gradual annual loss of a PV module power output over its life, typically around 0.5% per year for crystalline silicon.