Energy is an amount of work done, or the ability to do work, measured in Joules. Electricity is a form of energy, but is generally expressed in its own units of kilowatt-hours (kWh) where 1 kWh = 3,600,000 Joules and is the electricity supplied by 1 kW working for 1 hour.
Power is the energy converted per second, i.e. the rate of work being done, measured in watts (where 1 watt = 1 Joule/sec. and 1 kilowatt = 1000 watts).
Hydro-turbines convert water pressure into mechanical shaft power, which can be used to drive an electricity generator, or other machinery. The power available is proportional to the product of head and flow rate. The general formula for any hydro system’s power output is:
P = h r g Q H
Where:
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P is the mechanical power produced at the turbine shaft (Watts).
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h is the hydraulic efficiency of the turbine.
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r is the density of water (1000 kg/m3).
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g is the acceleration due to gravity (9.81 m/s2).
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Q is the volume flow rate passing through the turbine (m3/s).
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H is the effective pressure head of water across the turbine (m).
The best turbines can have hydraulic efficiencies in the range 80 to over 90% (higher than all other prime movers), although this will reduce with size. Micro-hydro systems (<100kW) tend to be 60 to 80% efficient.
If we take 70% as a typical water-to-wire efficiency for the whole system, then the above equation simplifies to:
P (kW) = 7 ´ Q (m3/s) ´ H (m) | 
