In short: micro-hydro power output is roughly power = density × gravity × flow × head × efficiency. A modest site — 50 liters/second of flow and 10 meters of net head at 70% combined efficiency — produces about 3.4 kW, or roughly 12,000 kWh a year at a conservative 40% capacity factor. Head matters as much as flow: doubling either one roughly doubles the power, and head is usually the cheaper of the two to add.
This is a planning-aid estimate for comparing sites and equipment. Water rights, environmental review, and turbine selection should be confirmed with your state water agency and a qualified small-hydro engineer.
How to calculate micro-hydro power output
Hydropower comes from water falling through a height (head) at a certain rate (flow). The available energy per second — power — is the weight of the falling water multiplied by how far it falls, captured by a turbine at some efficiency less than 100%:
This calculator converts your flow and head into SI units, multiplies by density, gravity and your chosen efficiency, and reports the result in kilowatts, watts, horsepower or BTU/hr. For a rough annual figure, it multiplies that power by the hours in a year and by a capacity factor reflecting how often your stream actually carries that flow.
Worked example. Take 50 liters/second (0.050 m³/s) falling through 10 meters of net head at 70% efficiency. Power = 1,000 × 9.80665 × 0.050 × 10 × 0.70 = 3,432 watts, or 3.43 kW. At a 40% capacity factor that is 3.43 × 8,760 × 0.40 ≈ 12,027 kWh over a year — about 33 kWh a day.
Water's density barely matters here. Unlike weight or buoyancy calculations, hydropower output is dominated by flow, head and efficiency — the density of water changes by well under half a percent across any realistic stream temperature, so this calculator uses a fixed 1,000 kg/m³.
Why head, flow and efficiency change the answer
Head is the vertical drop the water falls through before reaching the turbine. Gross head is the raw elevation difference between intake and turbine; net head subtracts the friction the water loses traveling down the penstock (pipe). A long, narrow, or rough pipe can eat up a meaningful share of gross head — always enter net head here, not the number you get from a map or altimeter.
Flow is how much water arrives per second. Power scales linearly with both flow and head, so a site with twice the flow — or twice the head — produces roughly twice the power, all else equal. Because head is usually fixed by geography while flow varies with the weather, sites with more head per unit of flow tend to be steadier producers.
Efficiency captures everything the water's energy has to pass through before it becomes electricity: the turbine runner, the generator, and any belts or gearboxes between them. Well-matched impulse turbines (Pelton, Turgo) on high-head sites can exceed 85%; low-head reaction turbines and screw generators typically land in the 60–75% range. Combined efficiency is always the single biggest assumption in a micro-hydro estimate.
Power output by flow and head
Estimated electrical power, in kilowatts, at a fixed 70% combined efficiency. Find your flow rate down the left, your net head across the top.
| Flow | 5 m head | 10 m head | 20 m head | 50 m head |
|---|---|---|---|---|
| 10 L/s | 0.343 | 0.686 | 1.37 | 3.43 |
| 25 L/s | 0.858 | 1.72 | 3.43 | 8.58 |
| 50 L/s (default) | 1.72 | 3.43 | 6.86 | 17.2 |
| 100 L/s | 3.43 | 6.86 | 13.7 | 34.3 |
| 200 L/s | 6.86 | 13.7 | 27.5 | 68.6 |
Worked examples
Backyard stream, crossflow turbine
1.02 kW
20 L/s and 8 m of net head at 65% efficiency (crossflow) produce about 1.02 kW — roughly 3,130 kWh a year at a 35% capacity factor, or about 8.6 kWh/day.
Farm creek, Kaplan turbine
8.83 kW
80 L/s and 15 m of head at 75% efficiency (Kaplan) produce about 8.83 kW — around 34,800 kWh a year at a 45% capacity factor, enough for roughly 3 average U.S. homes.
Mountain penstock, Pelton wheel
2.50 kW
Just 5 L/s but 60 m of head at 85% efficiency (Pelton) still produce about 2.50 kW — proof that head, not flow, is often the more valuable resource. That's about 10,950 kWh a year at a 50% capacity factor.
What else affects the result
Penstock and intake losses. Friction in the pipe, trash-rack losses at the intake, and bends in the pipeline all reduce net head below gross head. Longer or smaller-diameter pipe means more loss — this is exactly why the calculator asks for net, not gross, head.
Seasonal flow variation. Most streams run high in spring and low in late summer. The capacity factor in the advanced section is a single-number stand-in for that whole seasonal curve; a site with a small storage pond can smooth it out and support a higher capacity factor.
Transmission and inverter losses. If you are running power any distance to a house or battery bank, or through a grid-tie inverter, expect another few percent of loss beyond the turbine-generator efficiency modeled here.
Environmental flow requirements. Many permits require a minimum "compensation flow" to stay in the stream for fish and habitat — that portion of the flow is never available to the turbine and should be subtracted from your measured flow before entering it here.
Frequently asked questions
How much power can a micro-hydro system produce?
It depends entirely on your flow and head. A modest site with 50 liters/second of flow and 10 meters of net head produces around 3.4 kW at typical efficiency. Sites with more head or more flow scale up proportionally — doubling either one roughly doubles the power.
What counts as a good flow rate and head for micro-hydro?
Micro-hydro generally refers to systems under about 100 kW. Anything from a small garden stream (a few liters per second with tens of meters of head) up to a farm creek (50–100 liters/second with modest head) can be viable. Head is usually the more valuable resource, since even small flows can generate useful power with 20+ meters of drop.
What is the difference between gross head and net head?
Gross head is the raw elevation drop between the intake and the turbine. Net head is gross head minus the friction losses in the penstock (pipe) and intake screens. This calculator uses net head, since that is the head actually available to the turbine — always subtract your estimated pipe losses from the raw elevation drop before entering a value.
What efficiency should I assume for a micro-hydro system?
Combined turbine-plus-generator efficiency for small hydro typically runs 60–85%. Impulse turbines like Pelton and Turgo, suited to high head, reach 80–90%. Crossflow and Archimedes-screw systems, common at low head, typically run 60–75%. Use the preset buttons for a reasonable starting figure.
How much energy will my micro-hydro system generate per year?
Multiply the power output by the hours in a year (8,760) and by a capacity factor that reflects how often that flow is actually available. Run-of-river sites without storage typically see a 30–50% capacity factor, since flow drops in dry months. A 3.4 kW site at a 40% capacity factor produces roughly 12,000 kWh per year.
Do I need a permit or water right for a micro-hydro system?
In most U.S. states and many other countries, diverting water for hydro power — even a small stream — requires a water right and often environmental or fish-passage review. This calculator only estimates output; check with your state water agency or a small-hydro consultant before building anything.
Sources & references
- U.S. Department of Energy, Office of Energy Efficiency & Renewable Energy. Hydropower Basics.
- European Small Hydropower Association (ESHA). Guide on How to Develop a Small Hydropower Plant.
- U.S. Energy Information Administration. Electric Power Annual — residential electricity use figures.
John Mac, PE
Water Systems Specialist · Austin, Texas
Texas-licensed Professional Engineer with 13+ years in residential water systems and 180+ completed projects. Writes and reviews the calculators on watercalculator.us. Read full bio
Texas PE (2016) · BS Civil Engineering, Hydraulics (2013)