Along with increase in energy consumption and increasing importance of environment protection, replacement of nonrenewable and pollutant fossil fuels are more in focus. Solar energy is the most important renewable source of energy. Photovoltaic panels directly convert solar energy to electrical energy. One draw of photovoltaic panels is their low efficiency of energy conversion. This low efficiency gets worth as the panel temperature increases. Therefore it is necessary to keep the panel temperature as low as possible. To prevent the panel from getting warm, a fluid layer may be introduced on top of it. This layer will act as a spectrum filter and absorbs the undesirable infrared spectrum of solar radiation. In the present research, effect of such fluid layer on the performance of a photovoltaic panel has been experimentally investigated. Water and water based nanofluids of CuO, Fe 3 O 4 , and Al 2 O 3 were used as the spectrum filter and the effects of their volume fraction and their layer thickness on the performance of the panel has been investigated. The results showed that introducing a water layer of 2 cm thick on the panel leads to 44.22% increase in the panel efficiency as compared with a dry panel. Using a 0.025vol% of Fe 3 O 4 nanofluid increased the thermal efficiency of the panel by 21.05% as compared with the performance of the panel under water layer, but the electrical efficiency got worse. Considering that electrical efficiency of the panel is of more importance, one may conclude that a 2 cm thick water layer will be the best choice among the studied cases. In that case, the panel temperature would be 37.5% lower as compared with that of the dry panel. Keywords: photovoltaic panel, efficiency, cooling, nanofluid, panel surface temperature