The size of inverters in large grid-connected photovoltaic (PV) systems is increasing. At present, the largest installed inverters have nominal powers of several hundreds of kW, which makes their characterisation in the laboratory difficult. As an alternative, on-site characterisation requires only measurement equipment and has the advantage that the inverter is tested in its own PV system under real operating conditions. This paper describes an on-site testing procedure for assessing the power efficiency of an inverter and the results obtained for 13 inverters with nominal powers between 3.3 and 350 kW. The inverters were tested in PV grid-connected systems whose aggregate power capacity is nearly 150 MW. The energy efficiency of the inverters tested was also calculated yearly, and the results were compared with both European and Californian energy efficiency parameters, which are widely used in the current PV market. Copyright © 2010 John Wiley & Sons, Ltd.