The total efficiency of a co-generation system is commonly estimated using average heat and power demands. It is known that the estimated total efficiency greatly differs when using hourly average demand as opposed to annual demand data, owing to demand fluctuation. The aim is to determine the numerical values of the error when examining the total efficiency of co-generation system; and to explain analytically why the error arises. In the paper, based on mathematical model, the error is modeled as the ratio of the estimated total efficiency to the real total efficiency. The error was given by: (1) the heat to power ratios of the site and of the co-generation system itself; (2) the efficiencies of co-generation electric power and of auxiliary boiler; and (3) the demands fluctuation rates of the site. Estimated total efficiency is overestimated. The error achieves its maximum value when the heat to power ratio of demand and of the co-generation system match. The error is nearly constant, when a heat to power ratio at the site is higher than the heat to power ratio of the co-generation system itself. Such error varies by 10% when considering a gas turbine co-generation system, if demands fluctuate by 10% from their respective average demands.
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