The** power required** to drive a centrifugal pump is usually known as the** brake horsepower**. It can be expressed in terms of the water horsepower divided by efficiency.

In the metric system, **kilowatts (kW)** are used. Due to hydraulic, mechanical, and volumetric losses in a pump, the actual or water horsepower available for work on the fluid is less than the total horsepower supplied.

## Pump Efficiency

**Pump efficiency** is the ratio of the **water horsepower** delivered by the pump and the **brake horsepower** delivered to the pump shaft. When selecting a pump, a key concern is optimizing pumping** efficiency**. The energy usage in a pumping installation is determined by the flow required, the height lifted, and the length and **friction characteristics** of the pipeline. The power required to drive a pump is defined simply using SI units by:

where:

- P is the input power required (W)
- BHP is the brake horsepower
- ρ is the fluid density (kg/m
^{3}) - g is the standard acceleration of gravity (9.81 m/s
^{2}) - H is the net pump head added to the flow (m)
- Q is the flow rate (m
^{3}/s) - η is the efficiency of the pump