A program was written in Visual Basic for Applications (VBA) to generate the PV module I­V characteristic for a given irradiance and module temperature and then solve for the speed of the motor and flow rate of air in a given duct system. The code was initially applied to measurements of irradiance and temperature to calculate values of voltage and speed which were then compared to measurements. Furthermore, in order to compare the performance of both fans, the speed of each was simulated as a function of time of day for an imaginary irradiation profile. Extraterrestrial irradiation is calculated for May 17 in Edinburgh and the irradiation profile for that day is calculated as 80 % of extraterrestrial radiation. In the program, the length and other properties of duct are fixed and so the system is defined through the specifications of the PV module, the DC motor and fan characteristics. The program uses the hourly irradiance and temperature data to determine the PV I-V characteristic, which is then solved simultaneously with the motor equations to determine speed. The H-Q characteristic is then determined at the calculated speed.