Transformer Efficiency Calculator

A transformer efficiency calculation compares the electrical power delivered at the transformer output with the electrical power supplied at the input. The calculator produces Efficiency as a percentage and Power losses in kW.

Efficiency is used during transformer performance review, energy-loss analysis, facility load studies, and verification of test or metering data. The loss value can help account for the difference between upstream transformer input demand and downstream delivered load when reviewing feeder loading, service demand records, equipment heat loading, or operating-cost calculations.

This calculation uses measured, tested, or documented power values. It does not select a transformer, establish transformer kVA capacity, determine conductor ampacity, or certify installed field performance.

Power Basis and Measurement Conditions

Enter Output power and Input power on the same electrical basis and over the same time interval.

Output power is the real power delivered by the transformer to the connected load, expressed in kW.

Input power is the real power supplied to the transformer, also expressed in kW.

For a valid comparison, both values should represent the same operating condition. Do not compare an input reading taken during one load interval with an output reading taken during a different interval. A mismatch in demand level, load duration, metering location, or measurement method can produce a mathematically correct result that does not represent actual transformer efficiency.

The calculator reports the values as:

Result field	Meaning
Efficiency	Percentage of input power delivered as output power
Power losses	Difference between Input power and Output power
Output power used	Output power value entered into the calculation
Input power used	Input power value entered into the calculation

Efficiency Formula

Transformer efficiency is calculated as output power divided by input power:

\(\displaystyle \text{Efficiency} = \frac{\text{Output power}}{\text{Input power}} \times 100\)

Power losses are calculated separately:

\(\displaystyle \text{Power losses} = \text{Input power} - \text{Output power}\)

A result below 100% indicates that part of the incoming power is not delivered to the output load. The calculator expresses that difference as Power losses in kW.

Calculation Example

Enter:

• Output power: 98 kW
• Input power: 100 kW

Efficiency is:

\(\displaystyle \frac{98\text{ kW}}{100\text{ kW}} \times 100 = 98\%\)

Power losses are:

\(\displaystyle 100\text{ kW} - 98\text{ kW} = 2\text{ kW}\)

The resulting values are:

Result	Value
Efficiency	98%
Power losses	2 kW
Output power used	98 kW
Input power used	100 kW

The transformer delivered 98 kW to the load while drawing 100 kW at its input. The 2 kW difference is the calculated power loss for that measurement condition.

Electrical Use of the Loss Result

Transformer losses affect the power seen upstream of the transformer. If a secondary load requires 98 kW and the transformer input is 100 kW, the upstream source, primary feeder, and associated distribution equipment must supply the additional 2 kW of loss along with the delivered load.

The calculator result can support review of:

• Input-versus-output metering data
• Transformer energy-loss estimates over a known operating period
• Primary-side demand compared with secondary-side delivered real power
• Equipment-room heat loading associated with documented transformer losses
• Load-study reconciliation where transformer input and downstream load totals differ

Efficiency percentage alone does not establish primary current, secondary current, conductor size, AWG or kcmil selection, ampacity, raceway fill, voltage drop, or overcurrent protection requirements. Those decisions require the applicable system voltage, phase configuration, power factor where applicable, transformer ratings, conductor insulation temperature rating, terminal rating, adjustment factor, correction factor, current-carrying conductor count, and the governing electrical requirements accepted by the AHJ.

Field Verification Limits

Use Input power and Output power as real-power values in kW, not apparent-power values in kVA. A transformer’s kVA rating describes apparent-power capacity, while this calculation compares real input and output power.

The calculation does not separate core loss, winding loss, harmonic-related loss, no-load loss, load loss, meter error, or losses in conductors and equipment outside the selected measurement points. The location of each meter determines what is included in the reported Power losses.

A calculated efficiency above 100% indicates inconsistent source data, measurement basis, interval timing, unit conversion, or meter accuracy. Review the entered Output power and Input power before using that result in a load or energy analysis.