Power Supply Efficiency Calculator

Power Supply Efficiency

Of all the power a supply pulls from the wall, how much actually reaches the load as usable DC? That fraction is its efficiency. You compute it with η = P_out / P_in × 100%. Whatever the input gives up on the way out turns into heat somewhere inside the box. Take a 500 W load on an 80% efficient PSU: it draws 625 W from the outlet and throws away 125 W as heat.

Inside a switching supply (SMPS), the losses come from a few places. Switching losses cover gate drive and the brief overlap during turn-on and turn-off. Conduction losses live in the RDS(on) of the MOSFETs, the diode V_F and the ESR of the capacitors. Then there are magnetic losses: core hysteresis plus copper I²R in the transformers and inductors. A well-built resonant LLC or GaN-based design will sit at 90–95% over its normal load range. A linear regulator works differently. It's basically a variable resistor, so its ceiling is fixed at η_linear = V_out / V_in. Drop 12 V to a 5 V output and you'll never beat 41.7%.

Applications

This number shows up when you size mains breakers and UPS capacity, when you work out how much heat a server room has to shed, and when you check a supply against the 80 PLUS tiers (Bronze 82%, Silver 85%, Gold 87%, Platinum 90%, Titanium 94% at 50% load, 230 V). PUE in a data centre rides directly on PSU efficiency, and a battery-powered device uses the very same equation to estimate how long it will run.

FAQ

Why does efficiency drop at very low load? The fixed losses don't go away just because the load shrinks. Controller bias, gate drive and standby snubbers all keep drawing, so they eat a bigger slice of P_in. That's why the curve usually peaks somewhere around 40–60% load and falls off once you go below 20%.

Does higher efficiency always mean cooler operation? Yes. The losses are just P_in − P_out, and all of that becomes heat. At 500 W a 95% PSU only has to get rid of 26 W, while an 80% unit is stuck shedding 125 W.

Why are linear regulators still used? Because they run very quiet, react fast to load transients and put out essentially no EMI. On a sensitive RF or audio rail that's worth more than the efficiency you give up.