Transformers and Power Supplies
Power Supply Efficiency Calculator
Use Power Supply Efficiency when power-supply efficiency is the quantity you need. Enter DC output power and AC input power, then compare the answer with the equipment or operating limits that apply to your case.
Enter values for Power-supply efficiency
Do not mix values recorded under different conditions.
Preparing the calculation
Record waveform, frequency, load level, and winding or device temperature. If operating conditions changed between readings, calculate separate cases.
At least one field has a positive lower bound because it appears in a denominator. Check every prefix before entry; micro, milli, kilo, and mega are not interchangeable. For round-trip efficiency, use Battery Round-Trip Efficiency Calculator.
- DC output power
- Default example: 450 W. Enter DC output power in W.
- AC input power
- Default example: 520 W. Enter AC input power in W.
Before accepting the number
Unit-prefix mistakes often dominate the arithmetic error.
Trace an unexpected result back through the raw entries first.
Power-supply efficiency: calculation method
The displayed result follows efficiency = output ÷ input × 100. The entered quantities are DC output power and AC input power.
For the example shown, power-supply efficiency equals 86.54%. Treat that value as a demonstration, not a recommendation.
Calculate power-supply efficiency from DC output power and AC input power. If you also need transformer efficiency, continue with Transformer Efficiency Calculator.
Applying the result
Interpret Power-supply efficiency on the same basis used for the source values. Compare it with voltage stress, current stress, ripple, loss, regulation, and temperature.
Keep the input set beside the output so another reader can reproduce it. The next worksheet may be Lighting Wall-Plug Efficiency Calculator if wall-plug efficiency is needed.
Compare two operating cases
Change DC output power from 450 W to 540 W as the sole changed variable. The two results are 86.54% to 103.85%.
Use a separate scenario when multiple conditions change together.
What the formula leaves out
Efficiency varies with load and line voltage.
Outside the entered variables, consider isolation, creepage, waveform shape, and manufacturer operating limits. Test an additional scenario when an omitted effect has a plausible range.
Record waveform, frequency, load level, and winding or device temperature. For the companion motor efficiency calculation, open Motor Efficiency Calculator.
Common questions before using the result
What happens when a denominator is zero?
A zero denominator is undefined, so the affected field has a positive minimum.
How should I prepare the Power Supply Efficiency inputs?
Measured and rated values can represent different states. Identify RMS, average, peak, and rectified quantities explicitly.
What assumptions affect Power-supply efficiency?
Efficiency varies with load and line voltage. Also consider leakage inductance, parasitic capacitance, saturation, switching transients, and cooling.