Estimation of the Output energy of photovoltaic (PV) modules is difficult, because the output energy of PV modules installed Outdoors is influenced by various environmental factors. To estimate the Output energy, specifically the annual energy production from a PV module, a new method is proposed using contour maps centered on environmental factors. The output energy contour map estimated using the proposed method was quite similar to the experimentally measured contour map. The error ratios of the estimated annual energy production against that experimentally measured were lower than 3.7% and 7.6% for single crystalline and amorphous silicon PV modules, respectively. The results indicate that this method is useful to estimate the output energy of PV modules. (C) 2008 Elsevier BY. All rights reserved.