A simplified solar water heating model with proportional flow rate control, controlled by the production temperature, was developed. Yearly climate data and daily consumption load data were applied at hourly time steps. About 15 additional simulation days ensure a yearly periodic simulation. Model simplifications were validated through a time step dependency analysis, indicating that a one-hour time step results in a maximum deviation from the exact solution of about 2 %. The solar fraction increases, but its rate of increase decreases, with collector area. The solar fraction increases, but its rate of increase decreases, with storage volume. For low temperatures, the solar fraction increases with production temperature, up to a maximum above the consumption temperature; afterwards, the solar fraction decreases with increasing temperature. Increasing the collector area and decreasing the storage volume increases the temperature that maximizes the solar fraction. If the storage tank is adequately insulated, neglecting heat losses results in a maximum solar fraction deviation of around 2 %. Assuming heat transfer efficiencies of 100 % in the storage tank, when the actual efficiencies are of 75 %, results in solar fraction deviations of about 2 and 20 % due to production/storage and storage/consumption heat transfer, respectively. (C) 2017 Elsevier Ltd. All rights reserved.