If you have a good site, hydro-power is one of the most efficient and reliable forms of renewable energy. Micro systems tend to be between 60 and 90 per cent efficient – where efficiency measures how well the water’s power is converted into electrical power. Hydro also has good predictability, and in the UK output is generally greatest in the winter months when energy demand is highest.
In Hydro terms a good site generally means one on a steep hill with fast flowing water. The head (or maximum vertical drop) of water is key. Less than 10m is normally classed as low head, from 10-50m medium, and more than 50m, high. If the head and flow are good enough, useful power can be generated from small streams.
Somewhat surprisingly, old watermills are not necessarily suitable hydro electricity sites. Although they may have an old canal drawing water off the stream, many do not have a strong enough flow or high enough head to generate electricity effectively. This is because they were designed to operate machinery directly. It is estimated that up to 15 per cent may be suitable for upgrading.
If your site is suitable, it is also important that it is reasonably close to where the power is going to be used, and to a connection point to the grid, if you plan to export any surplus.
If you are off-grid, and have a good site, you can use power directly from the turbine, turning it on and off as you need it. Alternatively you can use batteries to store the excess.