Performance of photovoltaic (PV) modules is evaluated under the standard test condition, which rarely meets actual outdoor conditions. Environmental conditions greatly affect the output energy of PV modules. The impact of environmental factors, especially solar spectrum distribution and module temperature, on the outdoor performance of amorphous Si (a-Si) and multicrystalline Si (mc-Si) PV modules is characterized. The results show that the output energy of a-Si modules mainly depends on spectrum distribution and is higher under blue-rich spectrum. In contrast, the output energy of mc-Si module is sensitive to module temperature but not to spectrum distribution. (C) 2007 Elsevier B.V. All rights reserved.