Motor efficiency and load estimate calculator

Use this page when a motor record has measured electrical data or an entered input-power value and the project needs an estimate of shaft output, losses and load percentage. The calculator supports two input bases: measured current with voltage and power factor, or direct input power in kW.

The output is a project worksheet value. It does not choose a motor, decide duty suitability, set overloads, set starter parameters or replace manufacturer data.

Field use cases

The useful result depends on the quality of the entered values. A current reading without PF, an efficiency value from the wrong motor, or a rated output that does not match the nameplate can make the estimate look more precise than it is.

Input basis

For balanced three-phase measured-current records, the calculator uses line-to-line voltage. For single-phase records, it uses line-to-neutral voltage. The default values follow Australian 230/400 V, 50 Hz project context and can be edited where project data uses a different value.

Load interpretation

The percentage is an estimate of shaft output compared with rated output. It is not a thermal model, service-factor calculation or product capability statement.

Formula basis

For measured-current mode, the calculator first estimates input power:

text Pin_3ph = sqrt(3) x VLL x I x PF / 1000 Pin_1ph = V x I x PF / 1000

For input-power mode, the entered Pin_kW is used directly. The shaft output, loss and load percentage are:

text Pout_kW = Pin_kW x eta Loss_kW = Pin_kW - Pout_kW Load_% = Pout_kW / Prated_kW x 100

The efficiency value is entered as a percentage and converted to a decimal ratio in the calculation.

Worked records

Boundary with related calculations

Australian context

The defaults use Australian 230/400 V, 50 Hz context, metric units and local project terminology. The formulas are engineering relationships, not reproduced controlled standards tables.

Motor records can be affected by current standards, state or territory obligations, DNSP conditions, local authority requirements, project specifications, equipment ratings and manufacturer instructions. Keep the estimate with its input basis so another reviewer can repeat or replace the calculation when better data is available.

Stop points

• The input power value may be shaft output rather than electrical input.
• Current and power factor were not recorded under the same operating condition.
• The entered efficiency does not belong to the motor or duty being reviewed.
• The rated output is uncertain or does not match the motor record.
• The result is being used to set overloads, starter parameters or protection without device and manufacturer data.
• The review load has no documented project source.