The thermal performance of the PV/T (photovoltaic thermal) system and conventional collector is affected by water storage capacity. There are limited studies which show the effects of the storage capacity on the performance of different solar systems. This paper measures and analyzes the performance of a photovoltaic thermal system, conventional collector, and PV (photovoltaic) plate at different water storage capacities (25, 50, 75, 100, and 125 kg/m(2)). The cost payback period of the solar systems was calculated with the electricity price of South Korea (0.049 US$/kWh) and for the country with the highest electricity price of 0.364 US$/kWh (Australia). The thermal efficiency of the PV/T system and conventional collector increased sharply with increasing storage capacity. The cost payback period of the PV/T system and conventional collector reduced considerably with increasing storage capacity. Similarly, the cost payback period of PV/T system and PV plate reduced remarkably when electricity price increased from 0.049 to 0364 US$/kWh. The selection of the proper storage capacity could play a vital role for the performance enhancement and cost reduction in the PV/T system and the conventional collector. (C) 2013 Elsevier Ltd. All rights reserved.