Transformer Efficiency Calculator

Transformer efficiency is the ratio of useful output real power to input real power:

When using the loss-based method, input power is modeled as:

Copper and stray load losses are commonly scaled with load fraction squared: P_cu(x) = P_cu,FL × x², P_stray(x) ≈ P_stray,FL × x². Core loss is often treated as constant at fixed voltage and frequency.

1. Select a calculation method that matches your test data.
2. Enter output power using rated kVA, load fraction, and power factor, or enter real output power directly.
3. For the loss-based method, enter available loss components for your transformer.
4. Optionally enable temperature correction if winding temperature differs from reference.
5. Click Calculate to view results above the form, then export CSV or PDF.

1) Why efficiency matters in real installations

Transformer efficiency shows how much input energy becomes useful load power. Small percentage changes can mean large annual cost differences, because core loss exists whenever the unit is energized. Better efficiency reduces heat and can improve insulation life. For 24/7 service, no-load loss often dominates energy cost.

2) Output power versus nameplate kVA

Transformers are rated in kVA, but efficiency uses real power (kW). Real output depends on load fraction and power factor: kW = kVA × load × PF. Enter either rated kVA with PF or output kW directly to match your available data.

3) Core (no-load) loss behavior

Core loss comes mainly from hysteresis and eddy currents in the steel. It is tied to applied voltage and frequency and changes little with load. Light loading often lowers efficiency because this fixed loss dominates the input.

4) Copper loss and winding temperature

Copper loss is winding I²R loss and grows with the square of current. Resistance rises with temperature, so the same load can create higher copper loss when the transformer runs hot. Temperature correction helps compare operating cases to reference test conditions.

5) Efficiency versus load and the maximum point

Efficiency usually increases from light load to a peak, then falls as copper loss rises near full load. Maximum efficiency occurs when copper loss equals core loss. This guides sizing decisions: oversizing wastes energy at low load, undersizing increases heating.

6) Using standard test data

Open-circuit tests estimate core loss at rated voltage. Short-circuit tests estimate copper loss at rated current. Combining these values with an intended load level provides a practical efficiency estimate without full-load energy metering. For partial load, copper loss is commonly scaled by (load fraction)².

7) Power factor and non-linear loads

Low power factor reduces real output for the same kVA loading, which can reduce calculated efficiency. Harmonics from non-linear loads can add stray losses beyond simple I²R scaling. If harmonics are significant, use measured losses or include an allowance in “other losses”.

8) Practical reporting and improvement tips

Report efficiency with load, power factor, voltage, frequency, and temperature assumptions. Modern power transformers often exceed 98% efficiency near their design point, while smaller units may be slightly lower depending on losses. Improve performance by right-sizing units, maintaining tap settings, ensuring cooling airflow, and avoiding sustained overloads. Export CSV/PDF results for audits and commissioning reports.

1) What is transformer efficiency?

It is the ratio of real output power to real input power, usually expressed as a percentage. It indicates how much input energy is delivered to the load versus lost as heat.

2) Why can efficiency be low at light load?

Core (no-load) loss remains nearly constant whenever the transformer is energized. At light load, output power is small, so fixed losses represent a larger fraction of input power.

3) What losses are included in the loss-based method?

It combines core loss, copper loss, and optional stray or auxiliary losses. Core loss is mostly voltage-dependent, while copper loss scales approximately with the square of load current.

4) How does temperature affect efficiency?

Higher winding temperature increases resistance, raising I²R copper loss for the same current. If you have test losses at a reference temperature, applying correction improves comparability across operating conditions.

5) Do I need power factor to calculate efficiency?

If you enter rated kVA and load fraction, you also need power factor to convert to real kW output. If you already know real output power, you can enter kW directly without power factor.

6) Why might measured efficiency differ from the estimate?

Real systems may have harmonics, unbalanced loading, cooling differences, or additional stray losses not captured by simplified inputs. Meter accuracy and test conditions can also introduce differences.

7) Where does maximum efficiency usually occur?

Maximum efficiency occurs when copper loss equals core loss for the operating condition. Depending on design, this often happens between moderate and high load rather than at full load.