The determination of heat power required to design the heating systems in the greenhouses varies depending on the type, the equipment and the temperature requirements, the external climatic conditions and the management of the greenhouse. The projecting temperature required to determine the heat power in some places is considered to be the difference between the desired temperature in the greenhouse and the lowest average temperature that occurs consecutively at least twice in the last decade. This determined heat power is the required maximum heat power value which allows the whole year temperature to be kept at the desired value in the greenhouse. However, this determined maximum heat power is needed in a very short time of the year. For this reason, as a new approach, calculation of the heat power value by taking the hourly climate values may enable the heating system to be designed more realistically.