# How much energy does trimming a pump impeller save?

Canonical: https://senseible.earth/answers/pump-impeller-trimming-energy-saving

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](https://senseible.earth/answers/vfd-pump-fan-energy-co2-savings)
- [Borewell pumping energy](https://senseible.earth/answers/borewell-pumping-energy-per-kl)
- [Motor replacement savings](https://senseible.earth/answers/ie2-to-ie4-motor-replacement-savings)

Sources: [Bureau of Energy Efficiency, energy efficiency guides](https://beeindia.gov.in), [CEA CO2 Baseline Database, version 21.0](https://cea.nic.in/cdm-co2-baseline-database/?lang=en)

Last reviewed 2026-10-22.