A transformer has two kinds of loss. No-load losses occur in the core whenever the unit is energised, regardless of load. Load losses occur in the windings and rise with the square of the current. Over a thirty-year life the electricity they consume often costs more than the transformer itself, which is why the EU now regulates minimum efficiency.
Where the two losses come from
No-load loss, also called core or iron loss, is the energy needed to magnetise and demagnetise the core fifty times a second at 50 Hz. It depends on the grade of core steel, the lamination thickness and the flux density the designer chose — not on what the load is doing. It is present for all 8 760 hours of the year.
Load loss, or copper loss, is resistive heating in the windings plus stray losses in the tank and clamping structure. Because it follows the square of the current, a transformer at 50 % load produces only a quarter of its rated load loss. That is why the loading assumed in a comparison changes which offer wins.
Putting a number on it
The standard method is capitalisation: convert the annual energy cost of the losses into a present value and add it to the purchase price. In words, the annual loss energy equals the no-load loss times 8 760 hours, plus the load loss times 8 760 hours times the square of the average loading factor.
Take a unit quoting 600 W no-load and 6 500 W load loss, running at an average loading of 40 %. No-load contributes about 5 300 kWh per year; load loss contributes about 9 100 kWh. Over thirty years that is roughly 430 MWh. At any realistic industrial electricity price, a design with 20 % lower losses repays its premium well before the unit reaches mid-life.
What EcoDesign requires
Regulation (EU) 548/2014 sets maximum permitted losses for medium and large power transformers placed on the EU market. Tier 1 applied from July 2015 and the tighter Tier 2 from July 2021. Limits are given per rating and construction type, and for medium power transformers Tier 2 can alternatively be met through a minimum peak efficiency index.
The practical effect is that the cheapest possible design is no longer legal, and that loss figures on competing offers are now genuinely comparable. Every Protrafo transformer is built to meet or exceed EU EcoDesign Tier 2, with guaranteed loss values stated in the offer and verified in the routine test report.
How designers actually reduce losses
- Better core material — high-permeability grain-oriented steel, or amorphous metal for very low no-load loss.
- Thinner laminations and step-lap corner joints, which cut eddy currents in the core.
- A lower flux density, which means more core material and a heavier, larger unit.
- Larger conductor cross-section or transposed conductor, which cuts resistive and stray losses.
Every one of these adds material, mass and cost, and the two loss types trade against each other: lowering no-load loss usually enlarges the core, which lengthens each winding turn and raises load loss. The optimum therefore depends entirely on your loading factor, not on a general ranking of designs.
Ask for guaranteed no-load and load loss figures in watts on every quotation, then compare offers on capitalised cost rather than purchase price. If the unit will sit lightly loaded — a standby feeder, a solar inverter station overnight — weight the no-load figure heavily. If it runs near rating most of the year, load loss dominates the calculation.
