Photovoltaics Explained
18 minutes
Energy Payback Time of Solar Panels: Years to Repay the Factory Energy
Energy payback time of solar panels is how many years a system takes to make back the energy used to build it. The formula is EPBT = embodied energy / yearly energy output. Embodied energy is all the energy used to mine, refine, make, ship and install the system, in kWh. Yearly energy output is what the system makes in one year, in kWh, counted as the fossil energy it saves. Most embodied energy goes into polysilicon, ingots and wafers, with glass and the aluminium frame next. For modern silicon panels, studies find roughly one to two years in sunny places and longer in cloudy ones. Panels last 25 to 30 years or more. So a system pays back its energy many times over. The time keeps shrinking as factories use thinner wafers, less silver and less energy per kilogram of silicon. Sunshine matters as much as the factory: the same panel pays back about twice as fast where the sun gives twice the yearly energy. A related number, energy return on investment, divides lifetime output by embodied energy. After this Concept you can compute an energy payback time from two numbers and test a claim that panels never repay the energy used to make them.