Calculation sequence
Step-by-step method
- Convert mechanical horsepower to kW when I-P is selected.
- Divide synchronized shaft output by electrical input.
- Subtract output from input for loss power.
ElectricalIntermediateMotors and Drives
Motor Operating Efficiency Calculator
Calculate motor operating efficiency from measured electrical input and independently determined shaft output.
Why it matters: A defensible efficiency calculation requires synchronized input and output power at a clearly defined boundary.
Calculated result
Engineering evaluation
Operating efficiency (η)89.484 %
- Converted shaft output (Pᵒᵘᵗ,ᵏᵂ)
- 8.948 kW
- Calculated loss power (Pˡᵒˢˢ)
- 1.052 kW
Engineering interpretation
The entered operating point yields 89.48398% efficiency and 1.051602 kW calculated loss.
Important limitations
- This is not a certified efficiency test. Output-power method, uncertainty, drive losses, and manufacturer data require review.
Motor-driven equipment
Estimate electrical demand and energy use
Use nameplate information and an operating schedule to screen motor input power, current, annual energy, and operating cost. Results update automatically.
Choose the motor nameplate output-power convention.
Rated shaft output, not electrical input.
Used to estimate current; it does not change the hp-based energy result.
Use efficiency at the evaluated load point when available.
Use measured or defensible operating load rather than motor nameplate size alone.
Used only for the estimated current calculation.
Assumes all selected motors operate concurrently at the entered load.
Replace the profile with the actual occupied, process, or control schedule.
$/kWh
Use the applicable marginal utility energy charge; demand charges are not included.Estimated result
Total motor input power (Pin,total)6.21 kW
- Current per motor (I)
- 9.18 A
- Total coincident current (Itotal)
- 9.18 A
- Daily energy (Ed)
- 50 kWh/day
- Annual operating hours (Hy)
- 2,080 h/year
- Annual energy (Ey)
- 12,925 kWh/year
- Annual energy cost (Cy)
- $1,551/year
Equations and assumptions
Nameplate-based screening method
Horsepower is converted using 1 hp = 0.7457 kW. This estimate assumes steady average load and the entered operating-point efficiency. Voltage and power factor affect estimated current, but hp, load, efficiency, quantity, and schedule establish the energy estimate. VFD speed profiles, cycling, standby power, demand charges, and process variation require separate evaluation.
Technical basis: U.S. Department of Energy — Determining Electric Motor Load and EfficiencyEngineering knowledge center
Understand the calculation—not just the answer
Governing relationships
Pᵒᵘᵗ,ᵏᵂ = Pᵒᵘᵗ·0.745699872
η = 100Pᵒᵘᵗ,ᵏᵂ/Pᵢⁿ
Pˡᵒˢˢ = Pᵢⁿ−Pᵒᵘᵗ,ᵏᵂ
Current substitution
Pᵒᵘᵗ,ᵏᵂ = 8.948398 kW; η = 100×8.948398/10 = 89.48398%; Pˡᵒˢˢ = 1.051602 kW.
Equation legend
Inputs and calculated quantities
- Pᵢⁿ
- Electrical input power — kW
- Pᵒᵘᵗ
- Shaft output power — hp or kW
- η
- Operating efficiency — %
- Pᵒᵘᵗ,ᵏᵂ
- Converted shaft output — kW
- Pˡᵒˢˢ
- Calculated loss power — kW
Applications
How this calculation is used
Building design
Screen motor loss and efficiency relationships.
Construction
Check input and output measurement boundaries.
Commissioning
Calculate operating efficiency without substituting nameplate output for measured shaft output.
Quality controls
Assumptions and limitations
Assumptions
- Input and output represent the same stabilized operating point.
- Shaft output is independently determined rather than assumed from nameplate rating.
Limitations
- Not a certified motor-efficiency test.
- Output-power method, uncertainty, drive losses, and manufacturer data require review.
Field use
Verification procedure
- Follow the approved electrical-safety program and use correctly rated instruments.
- Confirm wiring, ratios, units, operating mode, and measurement boundaries.
- Document simultaneous readings and compare with approved project and manufacturer data.
Common engineering mistakes
- Combining values from different time intervals, phases, or measurement boundaries.
- Treating a screening result as equipment selection, protective-device coordination, or code compliance.
Technical basis
References and source standards
- NEMA Motors and Generators StandardsMotor rating, speed, efficiency, and operating context; verify the current applicable edition.
- IEEE Standards AssociationPower quantity, harmonic, transformer, motor, battery, and metering standards; verify the current applicable edition during professional review.
- NIST Guide for the Use of the International System of UnitsElectrical quantity, unit, symbol, and conversion context.
Engineering calculation disclaimer
These engineering tools are provided for educational, preliminary evaluation, field verification, and commissioning support. Results depend on the accuracy of user-entered information and the assumptions stated for each calculation. Every colored status and comparison is a screening indicator only; project criteria, contract requirements, applicable codes, manufacturer instructions, measurement uncertainty, and authorized engineering judgment govern. The tools do not replace project-specific engineering analysis or the judgment of a licensed professional engineer.
Read the full use and limitation statement