A photovoltaic thermal (PVT) system combines photovoltaic cells and thermal collectors and is capable of simultaneously producing low temperature heat and electricity. A complete PVT system requires a thermal as well as an electrical energy storages to meet any mismatch between the demand and generation. The objective of this work is to develop a methodology for effectively sizing a PVT system. A sizing philosophy, called the design space approach, is applied to size the overall system. A design space is the collection of all feasible design configurations. It is observed that the design space for a PVT system is governed by the electrical demand, the thermal demand, the temperature requirement for the thermal load, and the boiling point of working fluid. A sensitivity analysis of the overall system sizing is carried out by varying electrical load, thermal load, and thermal load temperature. (C) 2013 Elsevier Ltd. All rights reserved.