Calculating Horsepower

Water Horsepower

Water Horsepower is the amount of power applied to the water. It is related to both the flow rate and pump head. The flow is the flow rate going through a pump and it is commonly measured in gpm or MGD. Head is the difference in the elevation of the HGL (hydraulic grade line) between the pump inlet (suction) and the discharge.

Brake Horsepower

Brake horsepower is the amount of power that must be applied to the pump shaft in order to get the required water horsepower. It is related to water horsepower and pump efficiency. Some of the power applied to the pump shaft is lost to turbulence inside the pump casing (volute) and friction in the pump bearings and packing. The percentage of the power input to the shaft that is actually transferred to the water is the pump efficiency.

The formula for water horsepower is:

The formula for brake horsepower is:

Example:

How much horsepower is required to pump 350 gallons per minute with 45 feet of head?

If the pump is 85% efficient, what size motor is needed?