Transformer Efficiency Calculator

How it works

Output = kVA × load fraction x × PF · losses = P_core + x² × P_cu,FL · efficiency η = output ÷ (output + losses) · maximum efficiency when x² × P_cu = P_core → x = √(P_core ÷ P_cu) · all-day η = daily output energy ÷ (output + 24 h × P_core + copper energy)

A transformer has two kinds of loss. Core (iron) loss from magnetizing the core is almost constant whenever it is energized; copper loss in the windings grows with the square of the load current. Efficiency is therefore highest at the load where the two are equal — often 40–60% of rating for distribution transformers, which spend most of their life part-loaded. All-day (energy) efficiency counts the core loss during the hours with little or no load, which is why low-loss amorphous cores pay back on lightly loaded units. The losses come from the open-circuit and short-circuit tests or from the manufacturer’s data (IEC 60076, IEEE C57.12).

Worked example

A 500 kVA transformer with 1 kW core loss and 5 kW full-load copper loss supplies 75% load at 0.9 power factor: output 337.5 kW, copper loss 0.75² × 5 = 2.813 kW, total losses 3.813 kW, efficiency 337.5 ÷ 341.313 = 98.88%. Peak efficiency is 99.02% at √(1 ÷ 5) = 44.7% load. Running 12 hours a day at this load and idle the rest, the all-day efficiency is 98.59%, and the losses add up to 21,079 kWh a year.

Sources: Wikipedia — Transformer (energy losses and efficiency) · US DOE — Distribution transformers energy conservation standards (10 CFR 431)

Frequently asked questions