Water - power, head and flow rate

Online Hydropower Calculator

The calculator below can be used to calculate available hydroelectricity.

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 - Hydropower

The theoretically power available from a flow of 1 m3/s water falling 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

P = 1000 μ q g h         (2)


P = power available (W)

μ = efficiency (in general ranging 0.75 to 0.95)

Equation (2) can be simplified to express practically available hydropower in kW

P ~ 10 μ q h (kW)

Estimate Potential Energy saved in a Tank or Reservoir

You can estimate the potential energy saved in a tank or reservoir by dividing the volume in horizontal slizes as done in

You can copy the document to your Google Drive and use it as a template. 

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