Motor Efficiency Calculator – Input V, I, PF and Output Power

Direct current

η = (P / (I * U)) * %

Alternating single-phase 

η = (P / (I * U₀ * cosφ)) * %

Alternating two-phase

η = (P / (I * U * cosφ)) * %

Alternating three-phase

η = (P / (√3 * I * U * cosφ)) * %

Where：

η = Efficiency(%)

P = Power (W)

U₀ = Voltage [L - N] (V)

U = Voltage [L - L](V)

I = Current(A)

cosφ = Power factor

Use this tool to see how efficiently your motor is running. Enter voltage, current, power factor, and output to check if it’s wasting energy or needs replacement.

Motor nameplates list rated efficiency under ideal full-load conditions—but in the field, voltage imbalance, partial load, or aging can significantly reduce real-world performance. This tool uses your actual voltage (V), current (I), power factor (PF), and shaft output power to estimate true motor efficiency and flag anomalies.

Enter measured parameters to automatically calculate:

• Electrical Input Power (kW) – total power drawn from the supply

• Motor Efficiency (%) – ratio of mechanical output to electrical input

• Validity Check – alerts if efficiency >100% (indicating measurement error)

Supports single-phase, two-phase, and three-phase AC systems with real-time bidirectional calculation (you can also solve for unknown output power).

Key Formulas

Electrical Input Power (P _in):
• Single-phase: P _in = V × I × PF
• Two-phase: P _in = V × I × PF
• Three-phase: P _in = √3 × V × I × PF

Efficiency (η): η = (P _out / P _in) × 100%

Real-World Examples & Diagnostics

Example 1: Healthy three-phase pump motor
Measured: 400 V, 10 A, PF = 0.85, Shaft Output = 5.5 kW
→ P _in = √3 × 400 × 10 × 0.85 ≈ 5.95 kW
→ Efficiency = (5.5 / 5.95) × 100% ≈ 92.4%
Insight: Matches typical IE3 performance at ~75% load.

Example 2: Invalid measurement (common pitfall)
Measured: 230 V, 5 A, PF = 0.8, Shaft Output = 1.1 kW
→ P _in = 230 × 5 × 0.8 = 0.92 kW
→ Efficiency = (1.1 / 0.92) × 100% ≈ 119.6% ❌
Diagnosis: Efficiency >100% is physically impossible. Likely causes:
– Output power overestimated (e.g., using HP without derating)
– Current or PF measured incorrectly
– Motor not at steady state

Important Notes

• Efficiency cannot exceed 100% — values above indicate input error

• Use true RMS power analyzers for accurate V, I, and PF readings

• Always measure under stable, steady-state operating conditions

• Compare your result to IE-class benchmarks:

• IE1: ~70–85%

• IE2: ~85–90%

• IE3: ~90–96%

• Low efficiency may signal bearing wear, voltage imbalance, or undersized loading