Water - power potential, head and flow rate

Online Hydro-power Calculator

The calculator below can be used to calculate available hydroelectricity power.

density (kg/m3)


volume flow (m3/s)

head (m)

The theoretically power available from falling water can be expressed as

Pth = ρ q g h                      (1)


Pth = power theoretically available (W)

ρ = density (kg/m3) (~ 1000 kg/m3 for water)

q = water flow (m3/s)

g = acceleration of gravity (9.81 m/s2)

h = falling height, head (m)

hydropower, volume flow, head and power generation

Example - Hydro-power

The theoretically power available from a flow of 1 m3/s water with a fall of 100 m can be calculated as

P = (1000 kg/m3) (1 m3/s) (9.81 m/s2) (100 m)

  = 981 000 W

  = 981 kW


Due to energy loss the practically available power will be less than the theoretically power. Practically available power can be expressed as

Pa = μ ρ q g h                (2)


Pa = power available (W)

μ = efficiency (in general in the range 0.75 to 0.95)

Energy from Hydro-power

The potential theoretical energy in a volume of elevated water can be calculated

W = ρ V g h                      (3)


W = energy (J)

V = volume of water (m3)

Example - Energy in Elevated Water Volume

A water volume of 10 m3 is elevated 10 m above a turbine. The potential energy in the water volume can be calculated as

W = (1000 kg/m3) (10 m3) (9.81 m/s2) (10 m)

   = 981000 J (Ws)

   = 981 kJ (kWs)

   = 0.27 kWh

Potential Energy in a Tank or Reservoir

Energy water reservoir

You can estimate the total energy in a tank or a reservoir by integrating the potential energies for horizontal segments as done in 

Copy the document to your Google Drive or download it as a spreadsheet to make your own calculations. 

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