The availability of accurate performance figures for photovoltaic (PV) inverters is of prime importance when it comes to the selection of products, the design of installations or the estimation of the annual yield. This paper proposes an improvement of the current practise by introducing a new quantity to quantify the performance of inverters: the maximal achievable European efficiency. This quantity can be based on the distribution of the annual yield as a function of maximum power point (MPP) power and voltage or on the existing weighting factors of the European efficiency. In a first step, the concept of the required MPP voltage window is introduced on the basis of a detailed analysis of the operating conditions for a typical location in Central Europe. It quantifies the voltage window in which the inverter shall operate and in which the average efficiency shall be calculated. In a second step, a method to process the efficiency measurements as they are now commonly performed is described. By combining the detailed characterisation of the inverter efficiency and the distribution of the annual irradiation as function of the operating conditions, an average and a maximal efficiency over the whole inverter specified MPP voltage range are obtained. They provide a very valuable information about the overall performance of the inverter and about its sensitivity to the plant configuration. As a good compromise between simplicity and accuracy, the proposed concept of achievable European efficiency should be preferred to the current practises. Copyright © 2010 John Wiley & Sons, Ltd.