Of the many type of pump on the market, centrifugal designs are widely used to raise water against pumping heads up to about 25m (the height difference between the water table and tank's input pipe) multi-stage versions can cope with higher heads. A centrifugal pump has an impeller that throws water against its outer casing at high speed, the kinetic energy then being converted to a pressure head by an expanding output pipe. Centrifugal pumps are compact, robust, and well-suited to PV applications, but they are not normally self - priming and must therefore be kept submerged. This makes them suitable for pump/motors positioned below the water table. Alternative displacement or volumetric pumps including various self-priming types are more suitable for lower flow rates from very deep wells or boreholes.