Solar energy is mainly utilized via two routes--thermal and electrical. Two forms of energy are generated by two separate systems. The present work is about the study of a system which combines thermal and photovoltaic systems in one unit. The system is basically a conventional forced circulation type water heater. It is converted into a combined system by pasting solar cells directly over the absorber plate. The system equations are solved by a finite difference method. The simulations are done for different solar cell areas, mass flow rates and different water masses. The differential temperature controller, i.e. pump-off and pump-on, are used.It is shown that the pump-on time is more or less independent of the total stagnant water mass in the collector unit. The pump-off time is a sensitive function of the water flow rate. At higher flow rates, the pump is switched off early in the afternoon. Since the total working time decreases, the efficiency also decreases. There is an optimum flow rate for which the collector efficiency is a maximum. The cell efficiency, which is a function of temperature, is calculated using an iterative method. The average cell efficiency turns out to be more or less independent of the solar cell area on the absorber plate. This result helps in saving calculation time because the total electrical energy available for any solar cell area can be calculated simply. A normal domestic solar water heater of about 2 m 2 generates sufficient electrical energy (after taking into account the various losses in storage, etc. and the energy required by the pump) to run 2 tube lights of 20 W each for 5 h and 1 television of 30 W for 4 h.NOMENCLATUREA C = Aperture area of collector (m 2 )A S = Surface area of storage tank (m 2 )A c e l l = Solar cell area (m 2 )b = Flow tube spacing (m)C w = Heat capacity of water (J/kg)d = Internal diameter of tube (m)D h = Internal diameter of header (m)D p = Internal diameter of connecting piping (m)D s t = Internal diameter of storage tank (m)F R = Heat removal factorF l = Collector efficiency factorF = Friction factorg = Acceleration due to gravity (m/s 2 )h = Height between collector and storage tank (m)K f = Head loss coefficientL = Length of each tube on absorber plate (m)M w = Mass of water in tank (kg)m = Water mass flow rate (kg/s)N = Total number of tubes on absorber plate in collector arrayP = Ratio of solar cell area to collector areaΔP = Pressure drop (N/m 2 )Re = Reynolds numberS = Solar insolation (W/m 2 )T a = Ambient temperature (°C)T m = Storage temperature (°C)T o u t = Outlet collector water temperature (°C)Δt = Time interval (s)U L = Heat loss coefficient for collector (W/m 2 °C)u = Flow mean velocity (m/s)ρ = Density of water (kg/m 3 )δh t o t a l = Total head loss due to connecting pipes, tubes, bends, tees, etc.α = Absorptance of absorber plateτ = Transmittance of glass coverη c = Efficiency of solar cellη c o l = Collector efficiencyη P V T = Daily efficiency of (PV/T) system