Transformer Efficiency & Loss
Enter rating, core and copper losses and load to get efficiency, peak-efficiency load and annual loss cost.
Step-by-step
- Enter valid values to begin.
Model: core loss is constant and energised for 8760 h a year; copper loss varies with the square of load and applies only during the operating hours at the annual average load. Losses are treated as quoted at the same reference temperature, and regulation, harmonics, cooling-fan power and the small magnetising-current term are ignored. Efficiency bands (98% and 96%) are typical guidance for power transformers, not a standard limit.
Transformer Efficiency and Loss: Core, Copper and the Load That Pays Best
Core Engineering Principles
A transformer has no moving parts, yet it wastes power two ways. Core loss, also called no-load loss, is hysteresis and eddy current in the laminations. It depends on voltage and frequency, not on load, so it is there at 3 a.m. with nothing connected, every hour of the year. Copper loss, the load loss, is I²R in the windings plus stray loss, and it scales with the square of load.
Efficiency is output over output plus losses, and it peaks where the variable loss equals the fixed loss. Set core loss equal to x² times full-load copper loss and the load fraction is the square root of their ratio. The curve is shallow, so don’t chase the peak. The real money is in the no-load loss of a lightly loaded unit, and in copper loss that harmonics make worse, because winding eddy loss rises roughly with the square of harmonic order. Regulation, the secondary voltage drop from no load to full load, comes from winding resistance and leakage reactance and is a separate nameplate figure.
η = Pout / (Pout + Pcore + x² Pcu) • xmax = √(Pcore / Pcu)
Annual loss = 8760 × Pcore + hours × xavg² × Pcu
NEC & Standard References
IEC 60076-1 defines the rating and the no-load and load loss tests, and IEC 60076-20 sets energy efficiency of transformers, including peak efficiency index. DOE 10 CFR 431 sets minimum efficiency for US distribution transformers. IEEE C57.12.00 gives general requirements for liquid-immersed units, and IEEE C57.12.91 is the dry-type test code covering loss measurement. Check the adopted edition.1. Output: 500 × 0.75 × 0.90 = 337.5 kW.
2. Losses: 1200 + 0.75² × 5800 = 1200 + 3262.5 = 4462.5 W.
3. η = 337.5 / (337.5 + 4.4625) = 98.695%.
4. Peak efficiency at x = √(1200 / 5800) = 45.5% load (204.7 kW out, 2400 W loss), η = 98.84%.
5. Annual loss: 8760 × 1.2 + 8760 × 0.60² × 5.8 = 10,512 + 18,291 = 28,803 kWh, about $3,456 a year.
Loss is 0.89% of rating; the core alone costs about $1,261 a year, busy or idle.
- Sizing for the peak. A unit chosen for a two-hour peak spends the rest of the year lightly loaded, where no-load loss dominates.
- Reference temperature. Load loss is quoted at a stated winding temperature, commonly 75 °C or 85 °C depending on the standard. Compare quotes at the same reference.
- Harmonic loads. Rectifiers and drives add eddy loss, so a nameplate loss figure understates what you pay. Specify a K-rated unit.
- Parallel units. Two transformers sharing a load split copper loss but double the core loss. At low demand, switching one off can save energy.
- Price the losses. A cheaper unit with higher loss often costs more over twenty years.