The invention relates to a device (24) for protecting a stationary gas engine (12), said engine providing an electrical power (Pe) in response to a set power value (Pc) applied to the engine (12). The invention is characterised in that the device comprises measuring means (26) for measuring a quantity (IM) representing the combustion behaviour of the gas supplied to the engine, means (28) for determining an optimum electrical power value to be achieved in view of the measured quantity representing the combustion behaviour of the measured combustion gas, and means (30) for changing the set power value such that the electrical power supplied (Pe) is substantially equal to the pre-determined optimum electrical power value (Peopt).