The type of turbine, that converts the energy in falling water into shaft power, depends predominantly on the pressure head available and the design flow for the proposed hydropower installation.

Turbines are broadly divided into three groups; high, medium and low head, and into two categories: impulse and reaction. The head is the vertical distance that the water being used is allow to fall before being used to generate power.

Source: Micro-hydro Design Manual, IT Publications, 1993

Impulse turbines convert the kinetic energy of a jet of water in air into movement by striking turbine buckets or blades - there is no pressure reduction as the water pressure is atmospheric on both sides of the impeller. These are similar to conventional water wheels.

The blades of a reaction turbine, on the other hand, are totally immersed in the flow of water, and the angular as well as linear momentum of the water is converted into shaft power. As a result the pressure of water leaving the runner is reduced to atmospheric or lower.

The type of turbine systems chosen depends on the available head of water for many small scale installations a propeller turbine will be the best technology (suitable for heads of 1.5 – 5m) with cross-flow turbines being appropriate for higher heads of 3 – 20m. Other turbine types such as the Pump-as-Turbine, Pelton, Francis and Kaplan turbines are suitable for larger, low-head sites, but will often be too expensive for small schemes

Water turbines, like petrol or diesel engines, will vary in speed as load is applied or relieved. Although this is not such a problem with machinery that uses direct shaft power. However, this speed variation will seriously affect both frequency and voltage output from a generator. An electronic load controller (ELC) is used to control this variation by that continuously adding or subtracting an artificial load. This has the effect of ensuring that the turbine is permanently working under full load.