We propose a novel definition of efficiency, valid for motors in a nonequilibrium stationary state exchanging heat and possibly other resources with an arbitrary number of reservoirs. This definition, based on a rational estimation of all irreversible effects associated with power production, is adapted to the concerns of sustainable development. Under conditions of maximum power production the new efficiency has for upper bound 1/2 in situations relevant for mesoscopic systems. These results imply that at maximum power bithermal, stationary motors could reach a higher Carnot efficiency than the usual cyclic motors.