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Pump & Fan Speed Saving Calculator

Slowing a centrifugal pump or fan saves a surprising amount of energy, because power follows the cube of speed:P₂ = P₁ × (N₂ / N₁)³. Drop the speed by a fifth and you use roughly half the power. This works out what that is worth, and how long a drive takes to pay for itself.

The machine nowPower drawn at full speedKilowatts. Use the actual absorbed power if you have it, not the motor nameplate - a motor is rarely fully loaded.Running hours per year8,760 hours is continuous, all year.

The proposed changeReduced speedPer cent of full speed.System typeThe cube law only holds when the system is pure friction. Lift water to a height and part of the work never goes away, so the saving is smaller. This is the single biggest source of disappointment in drive projects.Drive lossesPer cent. An inverter is not free - typically 2 to 4 % of throughput is lost as heat in the drive.

MoneyUnit ratePence per kWh, from your own bill.Installed cost of the drivePounds, including installation. Leave at zero to skip the payback figure.

Estimated saving

Power at reduced speed -

Power saved -

Reduction -

Energy saved per year -

Saving per year -

Simple payback -

Read this before quoting the number to anyone. The cube law is a property of the pump, not of the system it sits in. If the duty involves lifting water to a height, a large part of the work is against static head and no amount of slowing down removes it - the saving can be less than half what the cube law predicts. The setting above is a rough allowance for that. The only way to know properly is the pump curve laid over the system curve.

Worked example

A 15 kW pump running 6,000 hours a year, slowed to 80 % speed, on a mixed system, with 3 % drive losses, at 26p per kWh.

15 × 0.80^2.5 = 8.59 kW, plus 3 % drive losses = 8.84 kW

15 − 8.84 = 6.16 kW saved

6.16 × 6,000 = 36,935 kWh, worth about£9,603 a year.

On the pure cube law the same case shows 7.09 kW saved and about £11,060 - nearly £1,500 a year of difference from one dropdown, which is why the system type matters.

An estimate for comparing options, not a guarantee. Real savings depend on the pump curve, the system curve, how the duty actually varies over the year, and whether the process will tolerate the reduced flow. Confirm with measured data before committing to a spend.