Power = 9.81 × flow (m³/s) × head (m) × efficiency. Even small streams can generate significant power.
A 10 m³/s flow with 50m head at 85% efficiency = 4,169 kW of power.
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Power (kW) = 9.81 × Q (m³/s) × H (m) × η. Where Q=flow, H=head, η=efficiency.