Two transformers with the same rating can have different purchase prices and different electrical losses. If your tender assigns values to those losses, compare the evaluated total owning cost (TOC) under the same specification and evaluation rules.
The evaluation formula
TOC = purchase price + A × no-load loss + B × load loss. A and B are the tender's monetary values per watt of no-load and load loss. Use the tender's own factors, currency, reference temperature, loss definitions and evaluation method. Do not substitute an assumed electricity tariff or loading profile when the tender supplies A and B.
No-load loss is present whenever the transformer is energised. Load loss changes with loading. Record the guaranteed losses for each design at the same rating and stated test conditions; compare both against mandatory limits before calculating TOC.
Worked comparison — example numbers only
All numbers in this paragraph and table are illustrative examples, not product data, a quotation or a tender factor. Suppose A is 2 illustrative cost units per watt and B is 1 illustrative cost unit per watt.
| Example design | Example no-load loss | Example load loss | Example loss adjustment |
|---|---|---|---|
| X | 100 W | 200 W | 2 × 100 + 1 × 200 = 400 cost units |
| Y | 80 W | 210 W | 2 × 80 + 1 × 210 = 370 cost units |
In this example, Y has a 30 cost-unit lower loss adjustment. If its quoted purchase price is no more than 30 cost units above X's price, Y has the lower evaluated TOC. Use your actual tender factors and actual offers to decide; this example makes no claim about TBDL product losses or prices.
Buyer comparison checklist
- Confirm both offers meet the same capacity, voltages, vector group, impedance, cooling, applicable standard and mandatory loss limits.
- Ask for the guaranteed no-load and load loss values and the applicable routine test documentation.
- Use the exact A and B factors and units in the tender. Include any other cost items only if the tender requires them.
- Calculate each TOC separately, then check the arithmetic and technical deviations before ranking.
- Request applicable type test reports and ISO certificates if the procurement documents require them; both are available on request for the relevant scope.
Sources
- U.S. Department of Energy, Distribution Transformers Energy Conservation Standards, proposed rule — defines A/B values and gives the TOC formula.
- IEC TS 60076-20:2017, Power transformers — Energy efficiency — official overview of transformer energy-performance evaluation methods; the specific TOC formula above is sourced from the DOE document.
For TBDL product loss tables, see S20, S22 and SCB18. See the buyer FAQ or send the technical specification for a project-specific response.