How to Estimate How Much Power You Have

A straightforward approximate equation to use is:

Power = Flowrate × Head × 6

kW = m3s–1 × m × 6

That is the easy bit. But first you need to work out these two things...

Head – The vertical distance from the surface of the water at the highest point at which you can access it (this might be the water level in an old supply to the top of a water wheel, or the surface level of water in a stream at the top end of your land) to the water level at the lowest point you can release it (perhaps the river water level at the point where it flows off your land, or the bottom of an existing wheel pit (in the case of an ex-mill site). Find this out in metres. Typically old mills sites have two to ten metres head, but hydro is often most economic with many tens of metres of head. There are machines which can use as little as one metre head. If you don't really have any head, and want to use the energy of the moving water in the river, this is something which is often asked about. “Is there something I can put into the stream to use the speed of the water to turn it?” The short answer is no, clearly you could get it to move something, but it is never worth it, unfortunately.

Flowrate – the amount of water you have. This is definitely the hard bit to work out. There are various methods to work this out, discussed also the BHA micro-hydro manual.

Once you have a flowrate, make sure it is in cubic metres per second (or “cumec”). One cubic metre per second is 1000 litres per second.

As a bit of an approximate guide (and I stress approximate), a small stream (e.g. that you can step across in one step) might have 10 litres per second (0.01 cubic metres per second). A big stream might have 100 l/s (0.1 cumec). A small river (the sort which might have a name on a map, but still be ‘wade across in a pair of wellies in the summer’ size) might be 1000 l/s (1 cumec). A decent river (the sort of thing that people know the name of, and if you had big trees on either side of, the branches wouldn’t meet) may have 10,000 l/s (10 cumec).

Stick the resulting figures into the equation at the top, and you have a rough output. Any output over 1kW probably could be exploited by some commercially available turbine. Options for finding out about what type of system is right for your site are either employ a consultant or hydro engineering firm (that’s us), or if you are really keen on doing it yourself as a project, a good suggestion is “Micro hydro design manual” ISBN 1 85339 103 4, a fairly comprehensive design manual for the technical side, along with the BHA guide for UK specific regulatory issues.