The electrical efficiency, eta(el), of cogeneration units (CGU) is a key factor for the profitability of electricity production from biogas. We measured the electrical efficiencies of eight biogas-driven CGUs on site over long time periods. The measurements show a decrease of eta(el) over engine lifetime with average rates of 0.2-0.7%(1) per 10,000 operating hours. The lowest rate of electrical efficiency decline was observed for an engine which was operated under a full maintenance contract, including a guarantee of electrical power output over lifetime as well as a complete overhaul. This indicates that plant owners should critically evaluate the cost-effectiveness of their strategy for CGU maintenance. For better integration of RES into the electricity market, biogas driven CGUs can be employed to provide balancing power. Thus engines have to be adapted to varying load conditions and more frequent starts and stops. Compared to full-load conditions, measured electrical efficiencies of the CGUs were 0.9-2.5% lower at 80% load and 2.7 to 5.0 % lower at 60% load. Our measurements indicate that the development of electrical efficiency with load is dependent on engine design and settings, rather than on absolute electrical power output. (C) 2017 Elsevier Ltd. All rights reserved.