Some thoughts about low power loads and power supply efficiency

December 4, 2020

I was recently reading another guide to power supply units (PSUs), which repeated the common advice that you shouldn't over-size your PC's power supply because its efficiency drops significantly at low loads. The general guideline I've read is that a PSU needs to operate at around half load for optimal performance, and it's common for a power supply's efficiency to plummet below 15% or 10% of its load. This time around, reading all of that made me suddenly twitch.

Both my home machine and my office machine have 550 watt PSUs, and they often draw only 40 to 60 watts. My home machine has to work hard to get about 150 watts of power draw, which is under 30% load. This means that I am definitely on the comparatively not great portion of the PSU loading curve.

(How bad it is is an interesting question. Both machines have the same PSU, certified as an '80 Plus Gold' efficient unit. Via Wikipedia, I discovered that certification reports are online here. This contains a test down to an input of 66 watts (giving 56 watts of output), at 84.6% efficiency. This seems not all that terrible to me.)

Does this matter? On the large scale of things, I think probably not. Almost all of the reported power draw from these machines gets turned into heat in the end (some of it turns into noise and air motion from fans). More or less PSU efficiency only changes where the heat is generated, and it may be better to generate the heat in the PSU if the PSU is better ventilated.

On the small scale of things, I would rather generate less heat in total while getting the same amount of work done as fast as before (at least during the summer). But at the same time, low power PSUs are apparently not a very popular market segment, so there aren't many options that are highly efficient (especially if you don't want to spend an arm and a leg). It's possible that my current PSU, oversized as it is, is still my most efficient option for machines that idle around 50 watts of power draw.

The corollary to this low load issue is that this means my power consumption numbers are not quite measuring what it looks like. When I measure an idle power draw of 66 watts for my work machine, that's the PSU's input power; it's actually outputting 56 watts to the motherboard, the CPU, and other components. If I changed the PSU to a higher efficiency one, the input power draw would drop although the power consumption of the components hasn't changed (and similarly for a lower efficiency PSU).

The measurement is fair and accurate in the sense that it's measuring the power usage of the entire system, including the PSU. But it means that I mostly can't make confident declarations about the relative power usage of various components of my machines, because the measured power draws are affected by both differences between PSUs and differences in PSU efficiency at various load levels.

(Since my home and office machines have the same PSU, one of these factors is eliminated in that comparison. But if I compare these machines to my previous set of numbers, the PSU is definitely a factor. My 2011 machines used the PSU supplied with the case, an Antec Neo Eco 620C, which was likely only 80 Plus Bronze and 82.7% efficient at a 10% load draw of 77 watts or so.)

Written on 04 December 2020.
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Last modified: Fri Dec 4 00:28:51 2020
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