The 30-Second Check
You have a stream nearby. Can it power your house, or is it just a pretty trickle?
A hydro power calculation answers that in about 30 seconds from two measurements: the head and the flow. How to measure them has its own page. Here we do the math and give you a verdict.
The Hydropower Formula
Falling water carries energy. Scientists call it potential energy: the bigger the drop, the more the water gives on the way down. Giant hydropower plants and a backyard turbine follow the same rule.
To calculate the maximum electricity your stream can produce:
Power in watts = net head (meters) × flow rate (liters per second) × 7
Head is the height the water drops from the intake to the turbine. Net head is what is left after friction in the pipe, which US farm guides say often takes about 20 percent.
Where does the 7 come from? The full formula is 9.8 (gravity) × head × flow × efficiency. Efficiency is the share of the water's energy that survives the pipe, turbine, generator and wires as electricity. The 7 assumes about 70 percent, the good end for small hydropower. Natural Resources Canada puts a micro hydro system at 50 to 70 percent, and the US Department of Energy works with about 53 percent for most systems, so multiply by 5 if you want a careful estimate.
Want the exact math, in meters or feet, with pipe loss and kWh a day? Use the hydro power calculator.
What Your Power Output Means
Under 200 watts, your stream cannot supply a home. A few LEDs, and that is it.
Between 200 and 1000 watts, you can run the fridge, lights and a laptop, but big appliances need backup.
Over 1000 watts (1 kilowatt), you are in business. A careful household can run on that.
Here is the part that surprises people. Hydropower does not stop at sunset. A turbine making 1000 watts day and night gives you 24 kilowatt-hours of electricity every day. To compare that with your own power requirement, add up your devices with the energy use calculator.
Four Real Examples
Stream A: 6 meters × 25 liters per second × 7 = 1050 watts. Whole-house territory.
Stream B: 2 meters × 40 liters per second × 7 = 560 watts. Useful, with solar or wind on top.
Stream C: 30 meters × 3 liters per second × 7 = 630 watts. Small flow, huge drop. A Pelton wheel would love this.
Stream D: 1 meter × 5 liters per second × 7 = 35 watts. Barely enough to charge a phone. Skip it.
How Much Head And Flow Is The Minimum?
As a rule of thumb, home hydro gets hard below about 2 to 3 meters of head or 2 liters per second of flow. They are not hard limits.
Low head gets ugly fast. To make 1000 watts at 1 meter of head, you need 143 liters per second. That is a river, and that amount of water means big pipes and lots of digging.
Low flow fails from the other side: below a certain amount of water, the turbine cannot push past the friction in its own bearings. Pico hydro can work with very little water, but only makes enough for cabin lights.
Do The Math For The Dry Season
This is the most important rule on the page: use the flow from the driest part of the year.
A stream that pumps 100 liters per second in March and 5 in August runs your house in March and barely a lightbulb in August. So size the turbine for the dry season, and let the extra water flow past the intake.
Ask neighbours who have lived nearby for years whether the stream ever runs dry, and plan for a drought year with solar or a generator as backup.
What Else Could Stop You
Even when the numbers work, two things can still kill a hydro system. In many countries you do not own the water on your land, so you need a permit before you divert any. And every meter of cable to your house costs money and loses some electricity, so a stream 500 meters away needs thicker cable or a higher voltage.
The Honest Answer
For most people, the answer is no, and solar or wind are the realistic options.
But with a real stream and a real drop, hydro is the gold standard of off-grid power generation. If your numbers are close, get a pro to redo the measurements, because moving the intake upstream can add head. Then check what a home hydro system costs before you decide.
Sources
- US Department of Energy, Planning a Microhydropower System (archived)
- Natural Resources Canada, Micro-hydropower systems
- University of Missouri Extension, Micro-hydropower site assessment
- Wikipedia, Hydroelectricity
Hard Words
| Term | What it means |
|---|---|
| Head | The height the water falls, in meters. Not the length of the stream. |
| Net head | The head left after friction in the pipe. Use it in your power math. |
| Flow | How much water passes each second, in liters per second. |
| Efficiency | The share of the water's energy that ends up as electricity. |
| Watts (W) | How much power the turbine makes at one moment. 1000 watts is a kilowatt. |
| Pelton wheel | A turbine with cups hit by a fast water jet. Loves big drops. |
Common Questions
How do you calculate hydro power?
Multiply net head in meters by flow in liters per second by 7 for a hopeful estimate in watts, or by 5 for a careful one.
How much energy does hydropower produce per day?
Multiply the watts by 24, because a turbine runs day and night. A 1000-watt micro hydro system can produce about 24 kilowatt-hours a day if the flow holds.
What is the minimum head and flow for home hydro?
As a rule of thumb, about 2 to 3 meters of head and 2 liters per second of flow. Below that, it rarely pays.