- This work experimentally assesses the thermodynamic performance of the only single kWe, variable-output continuous duty, internal combustion engine driven, residential micro-combined heat and power (mCHP generator (Marathon Engine Systems ecopower) available in the United States. The system underwent steady state testing over its full operating range, and first (energy) and second (exergy) law analyses were conducted on the collected data. At rated speed, first law results reveal the ecopower operated at an electrical efficiency of 24.4 +/- 0.7% and a utilization factor of 94.5 +/- 12.6%. At the same speed, the ecopower's second law electrical efficiency was 24.3 +/- 0.8% and total second law efficiency (including exergy in both the recovered heat an electrical streams) was 33.7 +/- 1.9%. This total second law efficiency was higher than that of common h sidential heating devices, including electric and gas furnaces and boilers, electric air-source heat pumps, and gas engine driven heat pumps. Further, the ecopower outperformed most of these devices at all part-load condition as well, indicating it would be an appropriate choice for a dispatchable generator to provide ancillary grit support services in a future with increased variable renewable generator penetration. System-level and intern. combustion engine irreversibilities are also presented to identify areas of inefficiency. The most prominet irreversibilities were (in decreasing magnitude) irreversible heat transfer, combustion irreversibility, friction and pumping losses, followed by generator and power electronic losses.