An approximate expression for the heat transfer rate of a heat recovery unit (HRU) fed by the exhaust gas exiting a microturbine operating at part-load is developed. The proposed expression can be used for various microturbine-HRU sets if the appropriate values of coefficients are used. The independent variables in this expression are limited to: the microturbine electrical power output, the circulating water mass flow rate, and the circulating water inlet temperature. By the method of non-linear regression the coefficients of the approximate expression are determined for two types of microturbines and five types of HRUs. The developed expression greatly facilitates the prediction of available thermal power. Thus it can be useful in mathematical modeling of cogeneration plants based on microturbines as well as at the design and operation of such systems.