A practical worksheet for checking units, reference conditions and annual loss costs.
By PowerTransformerB2B Editorial · Updated 2026-09-23
Start with a comparable pair
Before comparing two quotations, put the proposed capacity, voltage ratio, frequency and operating duty on the same sheet. A smaller transformer with a lower absolute no-load loss is not automatically the better choice. Record whether each number is a guaranteed quotation value, a catalogue entry or a measured test result. Keep those categories separate in the buying decision.
Separate the two loss fields
No-load losses occur while the transformer is energized. Load losses change with loading. Request both figures, their units, the rated configuration and the stated reference temperature. A brochure saying “low loss” supplies none of these numerical guarantees. Do not fill a missing value with zero or convert an efficiency percentage into an assumed loss without its loading and power-factor conditions.
Check watts before comparing kilowatts
Convert explicitly: 1,000 W equals 1 kW. Keep the original value and unit in your procurement worksheet. If the table heading and the numerical scale appear inconsistent, ask for a corrected datasheet. Do not silently repair the catalogue. A copied decimal-point error can change a lifetime-cost estimate far more than a small difference in purchase price.
Work through an illustrative calculation
For a simplified example, assume no-load loss of 1 kW, rated load loss of 8 kW, a constant loading fraction of 0.5 and 8,000 energized hours per year. Using a squared-loading approximation gives 1 + 8 × 0.5² = 3 kW, or 24,000 kWh per year. At an illustrative tariff of 0.10 currency units/kWh, that is 2,400 currency units per year. These are invented teaching inputs, not measurements or promises for a listed product.
Use your actual load profile
For variable demand, calculate time blocks separately and add their energy totals. Squaring the average load can understate the effect of peaks. Identify assumptions for harmonics, auxiliary cooling power, tariffs and outages. The directory calculator is an estimate, not a thermal design or acceptance test. Confirm the calculation method with the project engineer.
Put the guarantee into the RFQ
Ask every shortlisted manufacturer for the same loss schedule and reference conditions. Attach the proposal revision to the supporting report. Record any exception before commercial selection. Continue with the loss-cost calculator, documented product records and the RFQ checklist.