Transformer Loss & Efficiency

Estimate transformer efficiency at a selected load using no-load and rated-load loss data.

How this calculation works

Transformer losses are separated here into a constant energized no-load component and a load-dependent component that varies with current squared. Enter the apparent-power rating, the fraction of rated kVA being supplied, load power factor, core loss and rated-load copper/stray loss. The rated-load loss entry must exclude core loss to avoid counting it twice. Rating times load fraction times power factor gives active output. Adding the two loss components gives input power and allows output-to-input efficiency to be calculated. A load fraction of 0.5 means half rated kVA, not half rated kW independently of power factor. The estimate assumes rated voltage and frequency with loss data at a matching temperature, and does not model harmonics, cooling auxiliaries or temperature rise.

Inputs and units

Transformer apparent-power rating (kVA)
Load fraction of rated kVA (1)
Load power factor (1)
No-load core loss (kW)
Rated-load copper/stray loss (kW)

Method and formula

Pout = rated kVA × load fraction × PF; Ploss = Pcore + Prated-load × (load fraction)²; efficiency = 100Pout/(Pout + Ploss).

Worked example

Example inputs

Transformer apparent-power rating: 500 kVA
Load fraction of rated kVA: 0.5 1
Load power factor: 0.9 1
No-load core loss: 1 kW
Rated-load copper/stray loss: 5 kW

Calculation steps

Calculate active output: Pout = 500 × 0.5 × 0.9 = 225 kW.
Scale rated-load loss by load fraction squared: 5 × 0.5² = 1.25 kW.
Add constant core loss: total loss = 1 + 1.25 = 2.25 kW.
Calculate input power: Pin = 225 + 2.25 = 227.25 kW.
Calculate efficiency: 100 × 225/227.25 = 99.009901%.

Example results

Active output power: 225 kW
Estimated total loss: 2.25 kW
Input power: 227.25 kW
Efficiency: 99.00990099 %

Assumptions

Rated voltage/frequency and matched loss-test temperature
No-load loss is constant and load-dependent loss varies with current squared

Limitations

Does not estimate harmonics, temperature rise, inrush or cooling auxiliaries
Enter rated-load loss excluding core loss to avoid double counting

Sources

Schneider Electric: Specifying HV/MV Transformers at Large Sites: White Paper 258, p. 12: no-load losses, current-squared losses and load-dependent efficiency