Transformer Efficiency Calculator
Calculate transformer efficiency across load levels, including no-load and copper losses.
About Transformer Efficiency Calculator
Iron loss is constant while copper loss rises with the square of load, so efficiency peaks where the two are equal. That optimum is usually between 40 and 60 per cent of rating, which is why transformers are rarely sized for continuous full load. A transformer left energised at light load still burns its full iron loss, which over a year is often the larger part of the total.
Formula
Transformer Efficiency
η = output ÷ (output + iron loss + copper loss × load²); maximum efficiency at load = √(iron ÷ copper)
- iron loss
- Constant with load, present whenever energised
- copper loss
- Rises with the square of load current
Worked example
With the default values below, Transformer Efficiency Calculator returns Efficiency at That Loading of 98.8305%. Change any input above and every figure here is recalculated in your browser.
Inputs used
- Transformer ratingminimum 0.01 kVA
- 500 kVA
- Loadingaccepts 0–100 %
- 75 %
- No-load (iron) lossminimum 0 W
- 900 W
- Full-load copper lossminimum 0 W
- 5,500 W
- Load power factoraccepts 0.01–1
- 0.9
Results
- Efficiency at That Loading
- 98.8305%
- Total Loss
- 3,993.75 W
- Copper Loss at That Loading
- 3,093.75 W
- Loading for Maximum Efficiency
- 40.45%
- Annual Energy Lost at Continuous Duty
- 34,985.3 kWh
Step by step
- 1Values used
- 2Transformer Efficiency
- 3Efficiency at That Loading= 98.8305
- 4Total Loss= 3,993.75 W
- 5Copper Loss at That Loading= 3,093.75 W
- 6Loading for Maximum Efficiency= 40.45
- 7Annual Energy Lost at Continuous Duty= 34,985.3 kWh
Assumptions and limits
- The result is derived only from the 5 inputs listed above. Anything not among them is not part of the calculation.
- In particular, this calculator does not take inflation, tax or compounding frequency into account.
- Figures are rounded for display to 4 decimal places, so totals built from several rounded parts can differ slightly from the unrounded arithmetic.
How these calculators are built and tested is described on the Methodology page. If a figure looks wrong, please report it.
Frequently Asked Questions
How is Transformer Efficiency calculated?
η = output ÷ (output + iron loss + copper loss × load²); maximum efficiency at load = √(iron ÷ copper) Iron loss is constant while copper loss rises with the square of load, so efficiency peaks where the two are equal. That optimum is usually between 40 and 60 per cent of rating, which is why transformers are rarely sized for continuous full load.
Why does Transformer Efficiency matter?
A transformer left energised at light load still burns its full iron loss, which over a year is often the larger part of the total.
What values do I need to enter?
This calculator takes 5 inputs: Transformer rating, Loading, No-load (iron) loss, Full-load copper loss, Load power factor. The pre-filled defaults are a realistic starting point — replace them with figures from your own environment for a result you can act on.