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Cogeneration efficiency is electricity plus useful heat divided by fuel input, commonly 75-85%. The meaningful measure is primary energy saving: the fuel a separate power station at 50% efficiency and a boiler at 85% would need, less the fuel the cogeneration plant actually uses. Savings of 10-25% are typical.
Overall efficiency
Efficiency = (electrical output + useful heat) / fuel input
Primary energy saving
PES = 1 - fuel used / (electricity / reference power efficiency + heat / reference boiler efficiency)
Heat that is rejected to atmosphere does not count. Cogeneration only saves energy where there is a genuine, coincident heat demand — a plant sized for electricity with no heat sink is just an inefficient power station.
Many jurisdictions certify high-efficiency cogeneration against a primary energy saving threshold, often 10%. Certification usually requires metered electricity and heat data.