Tandem Solar Cell Energy Yield Under Real Outdoor Spectra

… more less Tandem solar cell energy yield is the energy a panel makes over a real year, not its efficiency under one test light. Outdoor sunlight keeps changing. Early and late in the day, and in winter, the sun is low, so light passes through more air and turns redder. Under cloud or haze the light is more blue and more scattered. The cells also run warm, often well above the test temperature, which lowers voltage. A two-terminal tandem is current-matched for the standard spectrum. At other times one subcell makes less current than the other, and the stack loses some output. Studies find this loss is real but fairly modest over a year, often a few percent. Design can shrink it. A slightly thicker top cell, a bifacial stack that takes light on its back face too, or a four-terminal layout that does not need matching all respond in different ways. So the number that matters is the yearly energy per installed power, in kWh per kW. Use it when you compare a tandem with a silicon panel at a given site. After this Concept you can explain why a tandem's yearly energy gain is smaller than its peak efficiency gain and name design choices that reduce the gap.