The amount of water power available for a particular site can be determined using the following formula:
The static head (or gross head) is equal to the vertical distance in metres between the water intake of the system and the point where the water enters the generator. For reaction turbines the static head includes the vertical distance from the turbine to the bottom of the draft pipe where the water is released.
The dynamic head is equal to the static head minus the friction head losses that occur in the pipework (penstock) between the water intake point and the hydroelectric generator, and is used to determine the amount of water power available.
The amount of water power that can be harnessed and converted into electrical power also depends on the efficiency of the turbine, and the efficiency of the electric generator.