A method to characterise the current total harmonic distortion for single-phase inverters is proposed. This method is based on the performance of the inverters along two different types of days: clear sky and partially cloudy sky days. An expression to estimate the average value of the current total harmonic distortion for each type of day is proposed. The values obtained from these expressions pick up the behaviour of the inverters for the two most usual climatic conditions. Moreover, the values obtained can be used to compare the electricity supply quality of different inverters. (c) 2004 Elsevier B.V. All rights reserved.