The greatest cost savings are often found in applications involving centrifugal pumps, because the energy savings made by slowing down a pump are determined by the Cube Law: reducing speed by 10% could reduce energy consumption by 25%, and 20% less speed could mean a 50% reduction.

You might think this sounds unlikely - surely a 10% reduction in speed should equate to a 10% reduction in energy usage? A little bit of background into how a pump works makes this more understandable.

A pump speeds up water flow: the faster it works, the faster the water travels, both into and out of the pump. As the water flow rate increases, so does the dynamic head of pressure that the pump has to overcome. Inside the pump is a set of vanes (called an "impellor") which accelerate the water from the centre of the impellor to the circumference, increasing both the flow rate and pressure. Therefore, slowing down a pump not only slows down the impellor, but also reduces the pressure against which it has to work. The combination of these factors means that energy-saving is related to the cube of the reduction in input speed (saving = reduction x reduction x reduction).