The energy requirements for the production of PV modules and BOS components are analyzed in order to evaluate the energy pay-back time and the CO2 emissions of grid-connected PV systems. Both c-Si and thin film module technologies are investigated. Assuming an irradiation of 1700 kWh/m2/yr the energy pay-back time was found to be 2·5–3 years for present-day roof-top installations and 3–4 years for multi-megawatt, ground-mounted systems. The specific CO2 emission of the rooftop systems was calculated as 50–60 g/kWh now and possibly 20–30 g/kWh in the future. This leads to the conclusion that in the longer term grid-connected PV systems can contribute significantly to the mitigation of CO2 emissions. Copyright © 2000 John Wiley & Sons, Ltd.