A simple approximation model is proposed to estimate the output power of a PV (photovoltaic) module coupled directly to a battery, given the array plane irradiation and the battery voltage. Calculations were performed on a PV streetlight system in the Netherlands; the PV modules of the system were made up of 40 cells. The average overestimation of the array output for the investigated periods is 6%, due to systematic errors at irradiation levels below 300 W/m² and at high module temperatures. Comparisons are made to a more elaborate calculation method based on a one-diode model for a PV module. The latter model gave a better accuracy of 2%. However, when pyranometer irradiation data were used instead of reference cell data, an overestimation of up to 10% for both models was found. It is therefore concluded that the error introduced by the approximation model may be acceptable in comparison with the uncertainty introduced by using pyranometer data.