The overall power efficiency of semiconductor heterostructure lasers operating well above threshold has been calculated in terms of the power output per unit widthP/W, the lengthl, the end reflectivityR, the absorption coefficient α and the conductivity per unit area σ. At currents sufficiently beyond threshold, the efficiency, optimised with respect to αl andR, depends only on the parameter(P/W)α/ηiE2gσwhere ηiis the internal efficiency and Egis the bandgap voltage. The form of this dependence and the range of reflectivity, laser length and power level for which it is applicable are derived. The results are compared with those which have been derived previously for lower relative power levels where optimum efficiency is obtained by operation reasonably close to threshold.