A Power Supply Is Least Efficient When Barely Loaded

Efficiency figures for power supplies are quoted as a single percentage. The reality is a curve, and the worst part of that curve is where much equipment actually sits.

Losses divide into two kinds

Some losses scale with the current being delivered, rising steeply as load increases. Resistance in windings and switching components accounts for most of these.

Others are essentially constant, present whenever the supply is energised. Control circuitry, magnetising current in the transformer and switching losses continue regardless of output.

Efficiency is the ratio of useful output to total input, so the fixed losses matter enormously when the output is small and hardly at all when it is large.

The curve peaks in the middle

At very low load, fixed losses dominate and efficiency collapses. A supply might deliver a small fraction of its rating while wasting a comparable amount.

As load rises, fixed losses become a smaller share and efficiency climbs quickly, typically peaking somewhere near half of the rated output.

Beyond that point, current-dependent losses grow faster than the output and efficiency falls again. Running a supply flat out is not its best operating point.

Oversizing costs more than it appears to

Choosing a supply with generous headroom seems prudent, and it does reduce stress. It also parks the unit permanently in the least efficient part of its curve.

A machine that idles most of the time and peaks rarely spends nearly all its hours at low load, so the idle efficiency dominates the energy actually consumed.

Certification schemes responded by testing at several load points including a very light one, precisely because a single mid-load figure concealed this.

Standby is a separate problem

A supply keeping a device ready to wake must run its control circuitry continuously while delivering almost nothing. Efficiency in any conventional sense is near zero.

Designs address this by switching to a different mode at very low load, firing in bursts rather than continuously and letting the output sag between bursts.

This is why some chargers emit a faint intermittent noise when nothing is connected. The audible pattern is the burst rate changing with the tiny load.

The waste appears as heat and ageing

Every lost watt becomes heat inside the enclosure, warming the capacitors that are the least heat-tolerant components present.

A supply working in its efficient range runs cooler and lasts longer, which compounds the argument for sizing it to the load rather than to a comfortable margin.

Efficiency figures therefore describe reliability as much as running cost. The two failure paths share a cause.