Effective electric efficiency (ℰEE) can be calculated using the equation below, where WE is the net useful electric output, ∑QTH is the sum of the net useful thermal output, QFUEL is the total fuel energy input, and α equals the efficiency of the conventional technology that would be used to produce the useful thermal energy output if the CHP system did not exist:
For example, if a CHP system is natural gas-fired and produces steam, then α represents the efficiency of a conventional natural gas-fired boiler. Typical boiler efficiencies are 80 percent for natural gas-fired boilers, 75 percent for biomass-fired boilers, and 83 percent for coal-fired boilers.
The calculation of effective electric efficiency is the CHP net electric output divided by the additional fuel the CHP system consumes over and above what would have been used by a boiler to produce the thermal output of the CHP system.
Typical effective electric efficiencies for combustion turbine-based CHP systems range from 50 to 70 percent. Typical effective electric efficiencies for reciprocating engine-based CHP systems range from 70 to 85 percent.