BecSpec Because specs matter.
Three-phase motors
Three-phase motors

Motor efficiency and running cost.

A one-percentage-point efficiency difference can be commercially meaningful in a motor that runs for thousands of hours a year — or almost irrelevant in one that rarely operates. The load point and the operating profile matter as much as the headline efficiency number.

Compare motors and estimate running cost →

BecSpec does not rank an incompatible motor higher because it is more efficient

Rated output, poles, voltage, frame, mounting and other compatibility evidence come first. Efficiency is used only after those requirements have been satisfied. The running-cost inputs in the finder do not alter inclusion, compatibility tier or result order.

What motor efficiency means

Motor efficiency is the fraction of electrical input power converted to mechanical shaft output. If a motor is delivering 7.5 kW at the shaft at 90% efficiency, its electrical input is approximately 8.33 kW before considering any wider system losses.

The simple relationship is:

electrical input kW = shaft output kW ÷ efficiency

That is why a small efficiency difference can accumulate into a meaningful energy difference when a motor operates continuously or for long annual hours.

Why load point matters

Efficiency is not a single immutable property across every operating condition. Australian Government guidance notes that electric motors are generally inefficient when operated at very low loads and are often most efficient nearer their normal operating range. Peer-reviewed replacement research similarly shows that economic conclusions can change when actual operating conditions are considered rather than relying only on nominal data.

For that reason BecSpec uses only the discrete efficiency points carried in the Australian GEMS source: 50%, 75% and 100% load. It does not interpolate a 60% or 85% efficiency value and does not substitute the full-load figure when the selected load point is missing.

How BecSpec estimates annual running cost

The finder asks for three operating assumptions: a selected load point, annual operating hours and an electricity tariff. For a motor with rated output P, selected load fraction L, efficiency η, annual hours H and tariff T:

shaft output kW = P × L

input kW = shaft output kW ÷ η

annual kWh = input kW × H

annual electricity cost = annual kWh × T

This is deliberately a motor electrical-input estimate at an assumed shaft load. It is not a whole-of-system model.

Worked example

Consider a 7.5 kW motor assumed to operate at 75% shaft load for 4,000 hours per year. At 90% efficiency, estimated shaft output is 5.625 kW and estimated electrical input is 6.25 kW. That is 25,000 kWh per year. At an electricity tariff of $0.25/kWh, the simple annual energy cost is approximately $6,250.

If another compatible motor has higher efficiency at the same 75% load point, BecSpec can calculate the difference in annual kWh and dollars using the same operating assumptions.

Comparing with an existing motor

An existing-motor efficiency value is useful only if it refers to the same operating load as the candidate comparison. A full-load efficiency should not be reused as though it were the existing motor's 50% or 75% efficiency.

BecSpec therefore labels the input explicitly as existing efficiency at the same load and expresses the efficiency gap in percentage points, not as an ambiguous percentage improvement.

Why BecSpec does not show generic payback in the result list

Payback requires a cost that belongs to the specific candidate and project. The incremental premium for one motor may differ materially from another, and an early replacement of a still-serviceable motor is a different economic decision from choosing between alternatives when replacement is unavoidable.

BecSpec therefore keeps the multi-candidate result list to energy use and dollar savings. Candidate-specific payback should only be shown where purchase and installation cost are explicitly attached to that candidate.

What the research says about replacement economics

The scientific literature supports a more careful decision than simply buying the highest efficiency class. A 2022 Energy study assessing 26 industrial motors found that replacement assessments based only on nominal conditions can produce incorrect conclusions; the authors compared different operating scenarios and cost treatments. A 2023 experimental study in Machines likewise evaluated motor replacement using energy, economic and environmental criteria across different operating conditions.

A broader life-cycle review published in the Journal of Cleaner Production in 2024 found that the use phase dominates the life-cycle impacts in the studies it assessed, while also noting that more efficient motor designs can involve different material burdens. That reinforces the value of examining real use rather than treating efficiency as an isolated badge.

The motor is not the whole system

Australian Government guidance emphasises that the largest savings can sometimes come from the driven system rather than from changing the motor alone. Pumps, fans, compressors, ducts, valves, throttling and variable speed control can dominate the system-level energy outcome. BecSpec's running-cost estimate intentionally excludes those effects so that a simple motor comparison is not presented as a plant-wide energy model.

What the V1 estimate excludes

  • time-of-use tariffs and demand charges;
  • power-factor penalties or correction;
  • variable load profiles between the 50%, 75% and 100% GEMS points;
  • pump and fan affinity-law savings;
  • VSD losses or VSD-driven system optimisation;
  • maintenance, downtime and reliability value;
  • heat recovery or cooling-load effects;
  • carbon pricing or future regulatory assumptions;
  • candidate-specific purchase and installation cost.

Compare Australian registered motors

In the BecSpec three-phase motor finder, first enter the compatibility specifications you know. Then open Estimate running cost to choose 50%, 75% or 100% load, annual operating hours and your electricity tariff. Existing efficiency at the same load is optional.

Open the motor finder & selector →

Evidence and technical sources

Decision support, not an energy audit

The estimate is intentionally bounded. It is useful for comparing otherwise suitable registered motors under the same assumptions; it is not a substitute for measuring the real load or modelling the complete motor-driven system.