Power is head times flow times efficiency
With water density ρ = 1000 kg/m3 and g = 9.81 m/s2, this becomes P (kW) = 9.81 × H × Q × η ÷ 1000, where H is net head in metres and Q is flow in m3/s. Head and flow trade off evenly: double either one and the power doubles. A high drop with a trickle can match a low drop with a torrent.
What common head and flow combinations give
| Head | Flow | Output (η = 0.6) |
|---|---|---|
| 1 m | 10 L/s | ~59 W |
| 5 m | 5 L/s | ~147 W |
| 10 m | 5 L/s | ~294 W |
| 20 m | 2 L/s | ~235 W |
| 50 m | 5 L/s | ~1.5 kW |
| 100 m | 10 L/s | ~5.9 kW |
Note 1 L/s = 0.001 m3/s. Real systems run 40–80% efficient once turbine, generator and pipe losses stack up; a good micro-hydro turbine reaches 60–80%. Multiply steady watts by 8766 for kWh per year at full uptime, then scale by a capacity factor of 0.3–0.5 for seasonal flow.
Where the efficiency goes, and which turbine fits
- System efficiency is turbine × generator × pipe. Turbines run 60–95%, generators 80–98%, pipe losses take 2–10%.
- High head, low flow suits a Pelton wheel (20 m and up, 80–90% efficient).
- Low head, high flow suits a Kaplan or crossflow turbine (2–20 m). Crossflow is the easiest to build and maintain.
- Runs around the clock, unlike solar or wind, so a small steady output adds up over a year.
Common questions
How much power can a small stream produce?
It depends on head and flow together. A drop of 5 metres with 5 litres per second yields roughly 245 W at 60% efficiency; 10 metres with 2 litres per second gives about the same. A few hundred watts runs lights and small devices, not a whole house.
What is the difference between gross head and net head?
Gross head is the raw vertical drop from intake to turbine. Net head subtracts pipe friction losses, usually 2 to 15%. Only net head belongs in the power formula, because friction never reaches the turbine.
Do I need a dam?
No. Run-of-river systems place the turbine in the flow with no dam, using a pipe to create head from an existing drop. They cost less and disturb the stream far less, which is why most small installations use them.
