Power Supply Efficiency Calculator
Input watts + output watts → efficiency η, heat dissipation, and 80 PLUS-style rating.
Result
- Input Pin500 W
- Output Pout425 W
- Efficiency η85.00%
- Heat dissipated75.00 W
- Heat as % of Pin15.00%
- 80 PLUS classification80 PLUS Silver-class (85%+)
Step-by-step
- η = Pout / Pin = 425 / 500 = 0.8500 = 85.00%.
- Heat = Pin − Pout = 500 − 425 = 75.00 W.
How to use this calculator
- Measure or read Pin from a wall-meter (Kill-A-Watt) and Pout from the DC rails.
- Enter both values.
- Read efficiency and 80 PLUS-equivalent rating.
About this calculator
Power-supply efficiency η = Pout/Pin tells you what fraction of the wall power becomes useful DC and what fraction becomes heat. The 80 PLUS certification program (launched 2004) defines tiers at 80%, 82%, 85%, 87%, 90%, 92%, 94% efficiency at various loads. Higher efficiency means less heat, smaller fans, lower cooling cost, and lower electricity cost — but at the price of higher upfront cost and (often) higher idle power for the extra control circuitry.
What this calculator does
This calculator reports DC-output efficiency η = Pout/Pin for a power supply or DC-DC converter, the corresponding heat dissipation Pin − Pout, and the equivalent 80 PLUS certification class. The 80 PLUS program is the industry-standard efficiency-rating ladder used in retail PSUs (Bronze 82%, Silver 85%, Gold 87%, Platinum 90%, Titanium 94% at 50% load), so the readout maps directly onto the labels on the box.
How it works — the formula
η = P_out / P_in
Heat = P_in − P_out
P_in_useful = η · P_in; P_in_wasted = (1 − η) · P_inConservation of energy: every watt drawn from the wall either ends up as useful DC output or as heat. There is no other significant loss mechanism in a sealed PSU. The 80 PLUS thresholds are measured at 20%, 50%, and 100% of rated load; this calculator reports a single operating-point efficiency, which is the most useful figure once you know your actual workload.
Worked examples
- Inputs:
- Pin = 500 W, Pout = 425 W
- Output:
- η = 85.0%; heat = 75 W; Silver-class
Typical 500 W PSU at near-full load.
- Inputs:
- Pin = 350 W, Pout = 315 W
- Output:
- η = 90.0%; heat = 35 W; Gold-class
50%-load operation often peaks efficiency.
- Inputs:
- Pin = 600 W, Pout = 432 W
- Output:
- η = 72.0%; heat = 168 W; below 80 PLUS
Common with unbranded or aged supplies — high heat and high electricity bills.
When to use this vs other tools
Use this for desktop / server / lab PSU evaluation. For battery-powered systems the more useful figure is energy efficiency over a duty cycle, not instantaneous η.
- kWh Cost per Month
Use to convert wasted-as-heat watts into a monthly electricity bill at your local utility rate.
- Ohm's Law
Use for the DC-side relationships (V, I, R) once you know the output power and rail voltages.
- Power (Watts)
Use to relate V and I to power, the underlying quantity in every efficiency calculation.
Authority note
80 PLUS is the industry standard for PSU efficiency ratings used in retail labeling. US DOE External Power Supply efficiency rules and the IEEE 1515 recommended-practice document are the regulatory and engineering complements.
Limitations
- Single operating-point efficiency. Real PSUs have a curve — measure at multiple loads (20%, 50%, 100%) for a full picture.
- Does not separate switching, conduction, and standby losses. For converter design, decompose those individually.
- Wall-meter input power includes power-factor reactive component; for accurate η use a true-power meter (Kill-A-Watt or Brand Electronics PowerScout).
- Aging: PSU efficiency drops a few percent over years as electrolytic capacitors degrade.
For commercial-grade efficiency claims (data-center, EnergyStar), follow the formal 80 PLUS or DOE testing protocol — point measurements are starting indicators only.