Two Numbers, One Verdict
Before you spend a single euro on a turbine, you need to know two things about your stream.
How much it falls.
How much it flows.
That is it. Once you have those two numbers, you can predict almost exactly how much power your stream can make. And you can figure this out yourself with a bucket, a stopwatch, and a length of garden hose.
Let's start with head.
What Head Is
Head is the vertical drop between two points on your stream.
Not the length of the stream. The drop. If your stream runs 100 meters but the ground only falls 1 meter over that distance, your head is 1 meter. Not 100.
Head is what gives water its punch. The further water falls, the harder it hits the turbine. A big drop with a small flow can make more power than a huge flow with no drop at all.
For a home hydro setup, you generally want at least 2 meters of head. Three is much better. Ten is great. Anything over 30 is mountain stream territory and a Pelton wheel paradise.
Measuring Head With A Hose
The simplest way to measure head is the hose method. You'll need a long garden hose and a friend.
Fill the hose completely with water. No air bubbles. Plug both ends with your thumbs.
Stand at the spot where you want your intake (the high point). Have your friend stand at the spot where you want your turbine (the low point).
Both of you let go of your ends at the same time. Water will flow from the high end. The hose acts like a giant U-tube. The water level inside the hose at the low end will sit exactly as high above the ground as the high end is above the low end.
Mark that water level. Measure from the ground to the mark. That is your head. To the centimeter.
This sounds silly but surveyors used to do something like it before laser levels were invented. It works because water always finds its own level.
For longer distances, you'll need to repeat the trick in stages. Walk the hose down the stream 20 meters at a time, adding up each stage as you go.
Measuring Head With A Phone
Your phone has a barometer. A tiny pressure sensor that detects how high you are above sea level.
Apps that read the barometer can give you altitude to the meter. Stand at your intake. Note the altitude. Walk to your turbine spot. Note the altitude. Subtract.
This is fast but not super accurate. The phone can drift by a meter or two depending on the weather. For a quick check it is great. For the real calculation, use the hose.
What Flow Is
Flow is how much water passes by per second.
Not the speed of the current. The volume. A wide slow river can have more flow than a narrow fast one.
Flow is measured in liters per second (or cubic feet per second in the US). A small home hydro setup wants at least 5 liters per second. A serious setup wants 30 or more.
The Bucket Method
The easiest way to measure flow is the bucket method. You'll need a bucket of known volume (10 liters works great) and a stopwatch.
Find a spot in the stream where you can catch all the water in one bucket. Often this is below a small natural waterfall, where the stream pours over a rock or log.
Hold the bucket under the falling water. Start the stopwatch. Stop when the bucket is full. Note the time.
Divide the bucket's volume by the time in seconds. That is your flow.
Example: a 10-liter bucket fills in 4 seconds. Flow = 10 / 4 = 2.5 liters per second.
For bigger streams, you'll need a barrel or a temporary dam. The idea is the same. Catch all the water in a known volume and time how long it takes to fill.
The Float Method For Bigger Streams
When the stream is too big to catch in a bucket, use a float.
Mark out a straight stretch of stream, exactly 10 meters long. Drop a stick or a small floating ball in at the top. Time how long it takes to drift to the bottom.
Measure the stream's average depth and width at three or four spots along that 10 meters. Multiply depth by width to get the cross-sectional area in square meters.
Then:
Flow = area × (length / time) × 0.85
The 0.85 is a correction factor. Water at the surface moves faster than water at the bottom (the bottom drags). The factor brings the average down to something realistic.
Example: a stream 2 meters wide, 0.3 meters deep, and a stick drifts 10 meters in 8 seconds.
Area = 2 × 0.3 = 0.6 square meters
Speed = 10 / 8 = 1.25 meters per second
Flow = 0.6 × 1.25 × 0.85 = 0.64 cubic meters per second = 640 liters per second
That is a lot of water. You'd be looking at a small-scale hydro plant, not a home setup.
Putting It Together
Once you have head and flow, you can estimate the power your stream can make.
Power in watts = head (meters) × flow (liters per second) × 7
Try it with a real example. A stream with 5 meters of head and 20 liters per second of flow:
5 × 20 × 7 = 700 watts
That is enough to run a fridge, lights, a laptop, and a TV, day and night. Forever.
→ Can your stream actually power your home? → The right turbine type for your numbers
One More Thing: Measure In The Dry Season
Streams are not constant. They roar in spring with melting snow. They trickle in August.
Measure your flow in the driest month of the year. That gives you the worst-case scenario, which is what you need to size your turbine. A turbine sized for spring flow will sit dry and useless all summer.
If you cannot wait for the dry season, ask neighbours how the stream behaves in late summer. Or check government water gauges if there is one anywhere near your stream.
