Transformer Efficiency Calculator (Iron and Copper Losses)
By Santaji GadepatilinUpdated October 27, 2026
Transformers lose a little energy as core (iron) loss whenever they are energised and as copper loss that grows with load. Efficiency depends on the balance between the two, as covered in this transformer efficiency calculation guide.
How to use the transformer efficiency calculator
Enter the rating, the load as a percentage, the load power factor, and the no-load and full-load losses from the test certificate or datasheet. You get the output, total losses, efficiency and the load at which efficiency peaks.
For more background, read Transformer Cooling Methods: ONAN, ONAF and Transformer Maintenance Checklist.
Transformer efficiency formulas
η = x·S·PF ÷ (x·S·PF + Pi + x²·Pc)
Maximum efficiency at x = √(Pi ÷ Pc)
Efficiency vs load (1,000 kVA, 1.4 kW core loss, 9.5 kW copper loss)
| Load | PF 1.0 | PF 0.9 | PF 0.8 |
|---|---|---|---|
| 10% | 98.53% | 98.37% | 98.17% |
| 25% | 99.21% | 99.12% | 99.01% |
| 38% | 99.28% | 99.20% | 99.10% |
| 50% | 99.25% | 99.17% | 99.07% |
| 75% | 99.11% | 99.01% | 98.89% |
| 100% | 98.92% | 98.80% | 98.66% |
| 110% | 98.84% | 98.71% | 98.56% |
Maximum efficiency occurs at √(1.4 ÷ 9.5) = 38% load.
Core loss vs copper loss
| Loss | Depends on | Present |
|---|---|---|
| Core (iron, no-load) | Voltage and frequency | Whenever energised, even with no load |
| Copper (load) | Current squared | Rises with load |
Because core loss is constant, a lightly loaded transformer runs 24 hours a day with relatively poor efficiency. Low-loss amorphous-core transformers cut core loss by 60–70%.
All-day (energy) efficiency
For distribution transformers, compare energy rather than power: total kWh delivered ÷ (kWh delivered + core loss × 24 h + Σ copper loss × hours at each load). This is how loss capitalisation and energy-efficiency standards rank transformers.
Frequently asked questions
How do I calculate transformer efficiency?
Efficiency = output ÷ (output + core loss + x² × full-load copper loss), where x is the load as a fraction of rating.
When is a transformer most efficient?
When copper loss equals core loss, at a load of √(core loss ÷ copper loss). For distribution transformers this is typically 35–55% load.
Why does copper loss vary with load squared?
Copper loss is I²R, and current is proportional to load.
What are typical efficiencies?
Large power and distribution transformers are 98–99.7% efficient at rated load. Small control transformers may be 90–96%.