What This Calculator Does
This hydro power calculator tells you how much electricity a stream can make, from two numbers: how far the water falls and how much water flows.
You get the power in watts, and how many kWh that makes in a day and a month. It also compares the day's energy with what an average US home uses. Not sure how to get your two numbers? Measuring head and flow shows you how with a hose, a bucket and a stopwatch.
How To Use This Calculator
- In Head, type how far the water drops from the intake to the turbine, straight down. Pick meters or feet in Head unit.
- In Flow, type how much water goes down the pipe each second. Pick liters per second, US gallons per minute or cubic meters per second in Flow unit.
- In Pipe friction loss, leave 20 if you typed the drop you measured on the land. If you already know your net head, type 0.
- In Overall efficiency, leave 50 unless your turbine maker gives you a better number.
- In Hours per day, leave 24. A turbine runs day and night while the water flows.
- Read the result card: the power, plus kWh a day and a month. The Breakdown shows net head, flow, the power in the falling water, and your share of an average US home.
The Formula, In Plain Words
Falling water pushes. The more water falls, and the further it falls, the harder it pushes.
The calculator first takes the pipe friction off your head. What is left is the net head: the drop the turbine really feels.
Then it multiplies four things: the weight of a cubic meter of water (1000 kg), gravity (9.81), the flow in cubic meters per second, and the net head in meters. That is the power in the falling water, in watts.
No turbine catches all of it. The turbine, the generator and the wires each lose a bit. So the calculator multiplies by the efficiency. At 50%, half of the water's power reaches your wires.
Power times hours gives energy. Watts times 24 hours, divided by 1000, gives kWh a day. A month is a year's worth of days divided by 12.
Feet and gallons work too. The calculator turns them into meters and cubic meters first. One foot is 0.3048 meters, and one US gallon is 3.785 liters, so 1 liter per second is about 15.85 gallons per minute.
A Worked Example
Take the starting values: a 10 meter drop, 20 liters per second, 20% pipe loss and 50% efficiency.
The pipe takes 20% of the 10 meters, so the net head is 8 meters. 20 liters per second is 0.02 cubic meters per second.
The water's power is 1000 × 9.81 × 0.02 × 8 = 1570 watts. Half of that reaches your wires: about 785 watts.
Running 24 hours, that is about 18.8 kWh a day and about 573 kWh a month. The average US home used 10,791 kWh in 2022, about 29.6 kWh a day, so this stream covers about 64% of it.
Now check it against a US guide. University of Wyoming Extension says that for a 53% efficient turbine, net head in feet times gallons per minute, divided by 10, gives watts. So 100 feet and 100 gallons per minute make 1000 watts. Type 100 feet, 100 gpm, 0% friction and 53% efficiency into this calculator and you get 1.00 kW.
What To Watch Out For
Streams change with the seasons. A stream that runs strong in spring can drop to a trickle in late summer. Use the flow from the driest month, and plan a backup for a drought year.
You don't always own the water on your land. In many countries you need a permit for home hydro before you take any water from a stream.
The pipe from the intake to the turbine is called the penstock. A wider, shorter pipe loses less of your head to friction. Read about the intake and penstock before you buy one.
This is an estimate. A real turbine works best at one head and flow, so ask your supplier for its own power curve. Then check what home hydro costs.
Hard Words
| Term | What it means |
|---|---|
| Head | How far the water drops, straight down. Not the length of the stream. |
| Gross head | The drop you measure on the land. |
| Net head | The drop left after friction in the pipe. The turbine feels this one. |
| Flow | How much water passes each second. |
| Efficiency | The share of the water's power that ends up as electricity. |
| Penstock | The pipe that carries water from the intake down to the turbine. |
| kWh | Kilowatt-hour: 1000 watts for one hour. Your power bill counts these. |
Common Questions
How do you calculate hydro power?
Multiply 1000 (the weight of a cubic meter of water in kg) by 9.81 (gravity), the flow in cubic meters per second, the net head in meters and the efficiency. The answer is in watts. With 8 meters of net head, 20 liters per second and 50% efficiency, that is about 785 watts.
What efficiency should I use for a micro hydro system?
University of Missouri and University of Wyoming Extension say 50% is a good rough estimate for most micro hydro systems. If your turbine maker gives a figure for the whole system at your head and flow, use that instead.
How much power can a small stream make?
It depends on the drop and the flow. A 10 meter drop with 20 liters per second makes about 785 watts after 20% pipe loss and at 50% efficiency. That is about 18.8 kWh a day, because a turbine runs day and night.
How do I convert gallons per minute to liters per second?
One US gallon is 3.785 liters, so divide gallons per minute by 15.85 to get liters per second. 100 gallons per minute is about 6.3 liters per second. The calculator does this for you when you pick gpm.
How much head loss does the pipe cause?
University of Missouri Extension says friction is often 20% of the gross head. A wider and shorter pipe loses less. If you already know your net head, set friction to 0.