How much energy does trimming a pump impeller save?

If a pump runs against a half-closed valve, trimming the impeller to the flow you need saves energy roughly with the cube of the diameter ratio. A 5% trim cuts power by about 14% on the affinity laws; measure real kW before and after.

When it fits

• The pump runs at constant flow below its design point.
• A discharge valve is throttled to hold flow.
• Load does not vary much; if it does, a VFD fits better.

Work it out

1. Measure kW, flow and head.
2. Size the trim with the pump maker's curve.
3. Saving kWh × 0.710 = kg CO2 saved.

Example: 30 kW pump, 6,000 hours

• 0.95³ = 0.857, so about 14% less power
• 4.3 kW × 6,000 h = 25,800 kWh
• 25,800 × 0.710 = 18.3 t CO2 a year

What Senseible does here

Senseible reads electricity bills, including kVAh, demand and solar export lines, into dated records with the units, factor and year behind every Scope 2 figure.

• VFD savings on pumps and fans
• Borewell pumping energy
• Motor replacement savings

Sources: Bureau of Energy Efficiency, energy efficiency guides, CEA CO2 Baseline Database, version 21.0

Last reviewed 2026-10-22.