Power Lost as a way to compare power supply efficiency

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via SilentPCReview:
http://www.silentpcreview.com/A_Better_ ... Efficiency

Try not to get too worked up in differences of 5 watts or 10 watts... but it's a very interesting read nonetheless, one of the few times I've seen the actual improvements in efficiency actually quantified between power supplies at a similar given load (rather than a as a percentage of power supply capacity, which is what individual reviews tend to do).

The difference between older less efficient units (often certified 80PLUS and 80PLUS Bronze) units vs. newer units (often 80PLUS Gold and 80PLUS Platinum certified) is quite marked. Makes me glad we finally retired the Seasonic S12 II/M12 II's from the System Guide...
Seems a pretty arcane way of saying the exact same thing. "Power Lost" isn't really a very informative metric. With load power and efficiency percentage, you have all the information you need. Of course, these need to be measured there and then.

If I just fire off Handbrake over 4 cores and Furmark over 2 GPUs, I'm showing 422 watts at the wall. I don't have the kit to measure what my PSU is actually delivering on the other end (but SPCR seems to), I work it out to be about 380 watts. So I can say "It delivers a load of 380 watts at 90% efficiency while drawing 422 watts from the wall" and I'm saying the same thing as SPCR here, doing so much more clearly, and giving enough information for the reader to derive any other figures he needs.

SCPR seems to want to emphasise the "power lost" term, but the correct term to be emphasising is the conversion efficiency - 90% in my case - while giving the input and output powers.
I think it's pretty useful actually.

It's relating the information in a way similar to how people talk about engine's putting out a certain amount of horsepower at the flywheel versus at the rear (or front or both) wheels at a certain RPM, and plotting it out at several points. You almost instinctively know you want the two numbers to be as close as possible, and I think it's easier to see that without percentages.

Makes it easy to see where one model may outperform another at a particular load, so if you know what your machine's pulling most of the time, you can see which unit does better in that range.

Also, I think the idea is that the reader doesn't have to derive anything; I don't know a lot of people who hear "you do the math" and go "YAY!" Most of them are on Ars, come to think of it. ;)
One thing that SPCR review does which most don't is take things down to really low wattage. Most reviews stay at 20% of rated efficiency or higher, in line with the 80+ rating scheme which also only does 20-100%.

If you're building a modern office PC with a simple i3/i5, iGPU and SSD (which I happen to have done twice in the last month) then your idle wattages are below 20% of the rated load of even a 300W PSU. So I was asking myself: "Should I buy a 300W bronze PSU or a 400W gold PSU?" This SPCR article provided decent information.
Hat Monster wrote:
Seems a pretty arcane way of saying the exact same thing.

It's saying the same thing, though I wouldn't call it arcane but practical.
I usually know roughly what a system I need a PSU for will consume. Let's say it's 50W idle and 300W under high load. Now I wanna know which PSU to take and with that load anything from 350W to maybe 550W could be reasonable, as there might be a 500W platinum PSU that's more efficient than the 350W ones. As their efficiencies are tied to relative load levels, I first need to see at what load level for the wattage class my system would end up.

350W => 14%, 85%
400W => 13%, 75%
450W => 11%, 66%
500W => 10%, 60%
550W => 9%, 54%

Now I can start comparing the PSUs.
If I would have given their efficiency at certain wattages, I could just look up which ones perform well on the wattages I need and whether the platinum 500W unit is better at the 50W I need than the silver 350W. If I get the difference in absolute wattage, I can even directly estimate how much of a difference this will make in kWh for a given usage profile. That's the number I get billed by.

This way one could argue that the important metric is excess wasted power or for AC'd homes excess heat losses, not conversion efficiency. I tend to think in conversion efficiency, too, though.

The suit in me tells me to look for absolute numbers, though, as every single Watt running 24/7 costs me two bucks a year and a PSU saving 5W on the idle load of my home server may be priced €10 higher if I run it three years.

In the end I am just happy if I get numbers for an absolute load in either way. Because then the main guess work, efficiencies at load points between measurements, is finally gone.
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