Machine Efficiency Calculator
Enter input and output power to get efficiency percentage and the watts lost as heat — the number your cooling and energy bill both care about.
Last updated: 2026-09-28
How the calculation works
- Efficiency is the output-to-input ratio; the difference is loss, and every lost watt becomes heat inside or around the machine.
- The result is bounded: output above input is physically impossible and the calculator rejects it.
- Per-unit output normalizes the figure for comparing machines of different sizes.
Formula
η = P_out / P_in × 100% P_loss = P_in − P_out
| Symbol | Meaning | Unit |
|---|---|---|
η | Efficiency | % |
P_loss | Power dissipated as heat | W |
Worked example
Interpreting the result
Efficiency is a load-point property, not a nameplate constant: motors peak near 75-100% load and can drop 10+ points below 50% load — oversized motors waste energy twice (poor efficiency and poor PF). Gear trains compound: two 97% meshes in series deliver 94%. When a machine runs hot, the loss figure tells you the heat load your cooling must remove — that is the engineering use of this number, not just the percentage.
Assumptions
- Steady-state power measurements at the same instant.
- Electrical input (W) or mechanical input — the ratio is unitless either way.
Limitations
- Measurement error matters: 5% instrument error on each side swings small efficiencies by double digits.
- For drives, add their losses: motor × gearbox × VFD efficiencies multiply.
Frequently asked questions
How is machine efficiency calculated?
η = useful output ÷ total input, ×100%. A pump absorbing 5 kW and delivering 3.5 kW of hydraulic power runs at 70% — the 1.5 kW difference heats the water and the pump.
Why do oversized motors waste energy?
Motor efficiency peaks at 75-100% of rated load; below 50%, efficiency and power factor both sag while magnetizing current stays constant — you pay for copper and iron losses that don't scale down with the load.