The size of the pool heater needed to effectively heat a pool is based on the size and shape of the pool as well as the climate. A swimming pool heater is rated according to its British Thermal Unit output.
One BTU can raise the overall temperature of 1 pound of water by 1 degree Fahrenheit. This means that a pool will be heated faster by a pool heater that produces a higher number of BTUs. The number of BTUs required to heat a pool is based on the pool's surface area, the degrees the temperature needs to rise and the heater's efficiency.