Power Supply Efficiency Calculator
Every power supply wastes some of the energy it draws, and efficiency is the single number that captures how much it keeps. This calculator takes the useful output power delivered to the load and the input power drawn from the source, then divides the first by the second and multiplies by 100 to give efficiency as a percentage. It also reports the power lost inside the supply, which is simply the input minus the output, dissipated as heat that the design must remove. The ratio is the standard way to compare supplies, size a heatsink, or estimate running cost: a 90% efficient supply turns nine tenths of what it takes into useful power and burns the rest. Enter your own output and input figures to rate a bench supply, check a vendor's efficiency claim at your real load, or work out how much heat a converter will produce in an enclosure. Efficiency is not constant; it changes with load and is usually best near the middle of the rated range, so measure at the conditions that matter to you. Every figure here is computed deterministically from the standard efficiency ratio, shown in full below, with a worked example that reconciles exactly to the calculator so you can follow each step.
Efficiency is output power divided by input power, as a percentage: Efficiency = (Pout / Pin) x 100. A supply delivering 45 W from a 50 W input is 90.00% efficient, losing 5 W as heat.
Power supply efficiency formula
Efficiency = (Pout / Pin) x 100
Pout = output power delivered to the load (W)
Pin = input power drawn from the source (W)
Power lost = Pin - Pout (W)
The output power divided by the input power gives the fraction of energy that reaches the load; multiplying by 100 expresses it as a percentage. The difference is lost as heat.
Worked example
A supply delivers 45 W to its load while drawing 50 W from the source.
- Power lost = 50 - 45 = 5 W
- Output / input = 45 / 50 = 0.90
- Efficiency = 0.90 x 100 = 90
- Efficiency = 90.00%
The supply is 90.00% efficient and dissipates 5 W as heat. These are the calculator's default inputs, so the result above matches the widget exactly.
Efficiency for a 50 W input
The same input power yields different efficiency depending on the output delivered.
| Output (W) | Efficiency | Loss (W) |
|---|---|---|
| 40 | 80.00% | 10.00 |
| 45 | 90.00% | 5.00 |
| 47.5 | 95.00% | 2.50 |
| 49 | 98.00% | 1.00 |
Method and measurement standards: US National Institute of Standards and Technology (NIST).
Power supply efficiency calculator: frequently asked questions
What is power supply efficiency?
Efficiency is the fraction of input power that reaches the output as useful power, expressed as a percentage. A supply drawing 50 watts from the wall and delivering 45 watts to the load is 90% efficient. The missing 10% is lost inside the supply, mostly as heat in switching devices, magnetics and resistive elements.
How do I calculate it?
Divide the output power by the input power and multiply by 100. With 45 watts out and 50 watts in, efficiency is 45 divided by 50, times 100, which is 90%. The power lost is the input minus the output, here 50 minus 45, which is 5 watts dissipated as heat.
Why does efficiency matter?
Higher efficiency means less wasted energy, lower running cost, less heat to remove and often a longer service life for components. For data centres and always-on equipment, even a few percentage points of efficiency translate into large energy and cooling savings over a year.
Does efficiency stay constant with load?
No. Efficiency varies with the load level and is usually highest somewhere in the middle of the rated range, falling at very light or very heavy loads. Quoted efficiency figures are measured at specific load points, so check the conditions when comparing parts.
What is a typical efficiency figure?
Modern switching supplies commonly reach 85% to 95% at their best operating point. Linear regulators are far lower, often well below 50%, because they burn the voltage difference as heat. Always read the datasheet for the figure at your actual load.
Official sources
- Measurement units, constants and electrical metrology: US National Institute of Standards and Technology (NIST). As at 25 June 2026.
Reviewed by the CalculatorHub team, edited by James Graham, 25 June 2026. See our methodology. This is general information, not financial, tax, legal or investment advice.