This paper describes a simplified dynamic thermal model which simulates the energy and overheating performance of windows. To calculate artificial energy use within a room, the model employs the average illuminance method, which takes into account the daylight energy impacting upon the room by the use of hourly climate data. This tool describes the main thermal performance (heating, cooling and overheating risk) resulting from proposed a design of window. The inputs are fewer and simpler than required by complicated building simulation programmes. The method is suitable for the use of architects and engineers at the strategic phase of design, when little data is available.