No-load losses, load losses and the real cost of a transformer over 30 years

A 630 kVA distribution transformer costs roughly as much as a mid-range car. Over its life it will convert several times that value into heat. Utilities have evaluated transformer losses in money for decades; industrial buyers rarely do. This article shows the method and why it changes purchasing decisions.

Two kinds of losses

• No-load losses P0 (iron or core losses) arise from magnetising the core. They are constant whenever the transformer is energised – 8 760 hours a year – regardless of the load.
• Load losses Pk (copper losses) are I²R losses in the windings plus stray losses. They vary with the square of the load: at 50 % load they are 25 % of the rated value.

The A/B capitalisation method

Each watt of loss is assigned a present value in euros. The buyer then compares offers on purchase price + A × P0 + B × Pk rather than on price alone.

A (€/W of no-load loss) = energy price × 8 760 h × annuity factor.
B (€/W of load loss) = A × (average load factor)².

Example assumptions: electricity 0.15 €/kWh, 30-year life, 5 % discount rate (annuity factor ≈ 15.4), average load 50 %.

• A = 0.15 €/kWh × 8.76 kWh/W·a × 15.4 = 20.2 €/W
• B = 20.2 × 0.5² = 5.1 €/W

Worked example: 630 kVA

	Offer 1 – Tier 2 limit values	Offer 2 – low-loss design
Purchase price	9 500 €	10 900 €
P0	540 W	430 W
Pk	4 600 W	4 600 W
Capitalised no-load losses (A = 20.2 €/W)	10 900 €	8 690 €
Capitalised load losses (B = 5.1 €/W)	23 460 €	23 460 €
Total cost of ownership	43 860 €	43 050 €

The dearer transformer is 810 € cheaper over its life – and the losses, at this load, are worth 3.5 times the purchase price. Note that the figures depend heavily on the load factor: for a transformer that idles most of the time (a rarely used backup feeder, a PV plant at night) the no-load term dominates; for a process plant at 80 % load the load-loss term does.

Practical conclusions

1. Never compare transformers on price alone – compare price plus capitalised losses.
2. For lightly loaded transformers, pay for low P0. For heavily loaded ones, pay for low Pk.
3. Do not oversize "to be safe": a 1 000 kVA unit at 30 % load has higher total losses than a 630 kVA unit at 50 %.
4. Ask for the routine test report – guaranteed values are only worth something if they are measured on your unit.

Our engineers run this calculation for you: send the expected load profile and energy price with your enquiry, and we return the rating and the variant with the lowest total cost, not just the lowest price.