Hydropower systems capture the energy of flowing water and convert it to electric energy by use of propellers or turbine wheels. Of course the potential for hydro-electric systems is depending on the availability of a suitable water flow, but where this resource exists it will provide cheap and reliable electricity.

In literature hydropower is divided in small and large hydropower systems, where it is not clear if small begins below 10 MW or 20 MW. In addition there are mini hydro (probably below 1 MW) and micro hydro (below 100 kW). 

However, small hydro power will have a significant contribution to future energy needs in wide areas of the world. It depends almost on proven and developed technology and has little maintenance.

The water flows via a channel or tube to a waterwheel or turbine where it strikes the bucket of the wheel, causing the shaft of the waterwheel or turbine to rotate. When generating electricity, the rotating shaft, which is connected to an alternator or generator, converts the motion of the shaft into electrical energy. This electrical energy may be used directly, stored in batteries, or inverted to produce utility-quality electricity.