The total system efficiency (ηo) of a CHP system is the sum of the net useful electric output (WE) and net useful thermal output (∑QTH) divided by the total fuel energy input (QFUEL), as shown below:
The calculation of total system efficiency evaluates the combined CHP outputs (i.e., electricity and useful thermal output) based on the fuel consumed. CHP systems typically achieve total system efficiencies of 60 to 80 percent.
Note that this measure does not differentiate between the value of the electric output and the thermal output; instead, it treats electric output and thermal output as having the same value which allows them to be added (kWh can be converted to Btu using a standard conversion factor). In reality, electricity is considered a more valuable form of energy because of its unique properties.