Let's say the measurement and calculation gives 175W. I've got 5 slots, so if I filled them all with 'typical' 5850s, I'll need 875W from the PSU purely on the 12V supply, ignoring the CPU and other loads.
PSUs are most efficient when operated somewhat below their rating, this also gives some margin for unforeseen circumstances. If you ignore the CPU,
you'll need a ~1200 watt PSU (875W / 0.75). 0.75 is a derating factor which keeps the PSU from running close to the limit.
http://www.anandtech.com/show/2624/3
My question is - at what point do I have to take the nature of AC into account? Is it only an issue when comparing 'power-from-wall' readings (in the USA, from 'kill-a-watt' type devices - in the UK I've not seen specific brand name power analysers, I just use Maplin power meters and extension multiplugs with power analysers built in)??
In theory, all AC power meters will report the same number that you are being billed for. Check with Maplin to see how well their meters compare to the utilities.
Residential AC is typically single phase, isn't it? Are the mains-electricity power meters measuring power the same way (volts x amps)?? If so, then the RMS issue comes into play, and you can't assume (in the UK) 240V times the current to be the power. Or can you?
Sine-wave alternating current and RMS voltage makes things annoyingly confusing when switching to DC output. Is this a complete non-issue, and when a PSU says their 12V rail will handle, say, 850W, and my mains power meter says I'm pulling 650W from the wall, I've got a nice fat buffer and am safe and efficient? Or is the rated 850W based off 'peak' power supplied in AC form, whereas in reality only RMS is continuously available to convert to DC, and my 'buffer' may not be anywhere near as large as I think?
I'd like someone who really knows the detail of AC conversion to DC, and how mains power meters measure, to educate me here... Haploid23 - you said that reading 1000W from the wall is a measurement of AC power... is that peak (non-continuous), needing an RMS calc, or are 'watts' always 'watts' regardless of AC, DC, single or three phase, etc??
I buy UPS backups for my servers, and have an emergency generator, and notice that *these* appliances often mess about with 'VA' ratings instead of good old *watts* - this suggests that 'watts are watts', and the 'VA' is just a scam to make the power supply seem more powerful (since it's going to be peak, not RMS, voltage - surely)??
But WTFDIK.
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This is where things fall off the rails. Since current and voltage are sinusoidal you can't simply multiply them unless it is a pure resistive load (it isn't.)
Maybe
http://en.wikipedia.org/wiki/Power_factor will help clear things up. If not, try
http://en.wikipedia.org/wiki/Volt-ampereIn general, your PSU manufacture will quote an efficiency number, typically the absolute highest measurement they can coerce out of a golden unit at the exact right temperature and age. For the sake of argument, we'll pick 80% efficiency. Your 875W to power the GPUs will draw ~1100W from the outlet.
