A rotary steam engine (RSE) is a simple, small, quiet and lubricant-free option for micro-cogeneration. It is capable of exploiting versatile thermal sources and steam temperatures of 150 to 180 ºC, which allow operational pressures less than 10 bar for electrical power ranges of 1 to 20 kW_e. An RSE can be easily integrated in commercially available biomass-fired household boilers. In this paper, we characterize the boiler-integrated RSE micro-cogeneration system and specify a two-control-volume thermodynamic model to conduct performance analyses in residential applications. Our computational analysis suggests that an RSE integrated with a 17 kW_th pellet-fuelled boiler can obtain an electrical output of 1.925 kW_e, in the design temperature of 150 ºC, the electrical efficiency being 9% (LHV) and the thermal efficiency 77% (LHV). In a single-family house inFinland, the above system would operate up to 1274 h/a, meeting 31% of the house’s electrical demand. The amount of electricity delivered into the grid is 989 kWh/a. An economic analysis suggests that incremental costs not exceeding € 1,500 are justifiable at payback periods less than five years, when compared to standard boilers.