Transformer Efficiency
Calculator
Calculate transformer efficiency at any load level and all-day efficiency. Enter the transformer kVA and loss data or select a standard size to auto-fill typical values.
Understanding Transformer Efficiency
No-load (core) losses occur continuously regardless of loading. Load-dependent (copper) losses scale with the square of the load current. Both must be minimised for an efficient transformer — DOE 2016 mandates minimum efficiency standards for distribution transformers.
At 50% load, copper losses are 25% of their full-load value (0.5² = 0.25). At 75% load, they are 56%. This quadratic relationship means lightly loaded transformers lose proportionally less to copper losses but proportionally more to no-load losses.
Maximum efficiency occurs when no-load losses equal copper losses at that operating point: optimal load = √(core losses / copper losses). For most units this is 50–70% of rated capacity. Choosing transformer size to match this is ideal for base-load applications.
If a transformer carries load for only part of the day, the idle core losses hurt its 24-hour efficiency. All-day efficiency captures this: a 100 kVA unit energized 24 h but loaded for only 8 h will have a lower all-day efficiency than one carrying steady 75% load around the clock.
No-load losses (also called iron or core losses) occur continuously whenever the transformer is energized, even at zero load. They result from hysteresis and eddy currents in the magnetic core. Under DOE 2016 standards, no-load losses for a 75 kVA unit are approximately 270 W.
Copper losses (also called load losses or I²R losses) occur in the transformer windings due to current flow. They scale with the square of the load: at 50% load, copper losses are 25% of full-load copper losses. At 100% load, they equal the rated copper loss in watts.
Maximum efficiency occurs when no-load losses equal the copper losses at that load level: L_opt = √(Pcore / Pcu). For most distribution transformers this is around 50–70% of rated load. Operating a transformer at or near this point minimises total losses per unit of output power.
All-day efficiency accounts for the fact that transformers are energized 24 hours but may not carry load for all of them. Because core losses are constant, a lightly loaded transformer over a long period has worse all-day efficiency than one running at a higher load factor for fewer hours. It is the ratio of total daily energy output to total daily energy input.
Comparing Transformer Efficiencies?
Use our Savings & Payback Calculator to see the annual energy cost difference between two transformers — and calculate how quickly a premium-efficiency unit pays for itself.