Power supply efficiency?

[DellAxim]

I bought one of those "Kill-a-Watt" power meters recently and hooked it up to my computer. I have an old 400w power supply (athlon dual core) and haven't seen the computer use more than 220 watts even under full load.

At the same time I'm thinking about upgrading and turning this into mini-itx box, with a DC-DC power supply. Obviously those are usually lower wattage. I'm just curious, if my power supply is sucking 220 watts from the wall, how many watts is the computer actually using? How efficient is a normal ATX power supply?

For example...would a power supply rated at 200 watts be enough for this computer? They're rated by OUTPUT, not INPUT, right?

 

[Dangman]

I bought one of those "Kill-a-Watt" power meters recently and hooked it up to my computer. I have an old 400w power supply (athlon dual core) and haven't seen the computer use more than 220 watts even under full load.

First and foremost: Take those KAW readins with a grain of salt. They're inaccurate:

Yes and a quick search would turn up this topic a million times over. Here is the recap:

1) APFC can fool Kill-A-Watts into giving you abnormally low readings (some times giving better than 100% efficiency)

2) Power supplies derate with temperature anywhere from 2w/c above a nominal rated at value to 10w/c.

3) Kill-A-Watt's and most power meters sample too slowly to catch transient loads (the Transient load from our tests is 117w and is COMPLETELY missed by Kill-A-Watts).

4) Power supplies last longer if you stay in the 40% to 60% range of their output.

5) power supplies are quieter if you stay in the 40% to 60% range of their output.

6) Power supplies are cooler if you stay in the 40% to 60% range of their output.

The power meters in UPS software are just as bad. You have to spend some change before you get anywhere near an accurate power meter when your PSU has APFC.

Here's Paul Johnson's post about the inaccuracy of the Kill-A-Watt:
http://hardforum.com/showpost.php?p=1032190998&postcount=7

I'm just curious, if my power supply is sucking 220 watts from the wall, how many watts is the computer actually using? How efficient is a normal ATX power supply?

The efficiency for most good quality PSUs these days is about 80%. However since you have an old PSU, that efficiency rating could be significantly less.

For example...would a power supply rated at 200 watts be enough for this computer? They're rated by OUTPUT, not INPUT, right?

Well can't answer that question because you haven't told us the specs of the PC (including the PSU) since we've now established that the KAW is inaccurate. I don't understand what you mean by output and input here.

EDIT:
Lets, for the sake of argument, assume that 220W figure is correct. Let's also assume that the PSU is 80% efficient. That would mean that PSU is actually outputting 176W of power. Now assuming again that all of that 176W is drawn from +12V rail, a 200W PSU with 18A on the +12V rail would technically be enough to power that system. But that PSU would definitely be pushed to the limits.

Here's another problem: Look at items 4-6 of the above list:. A 200W PSU with that kind of power draw would not be in the optimal thermal, longevity, and noise range for a PSU. To get in that optimal range, you'd need a PSU with at least 300W or 25A on the +12V rail.

The above was a best case scenario assuming that those KAW figures were accurate and that your PSU is 80% efficient. Many older PSUs tend to be 60% to 70% efficient

 

[DellAxim]

Lets, for the sake of argument, assume that 220W figure is correct. Let's also assume that the PSU is 80% efficient. That would mean that PSU is actually outputting 176W of power.

That's what I was getting at. If I had to guess, I'd say I have no APFC, the kill a watt says I have a power factor of .67. The PS is old enough that I wore out the fan a few years ago.

Also when I say watts, I mean VA. The KAW numbers "seem" pretty accurate to me, at idle it's about 80va, as I type this 91va. At 100% CPU load it's about 180va, it doesn't go over 200 until the DVD drive kicks in. I will probably get rid of that, some fans, and an extra HD.

I was thinking about a PS similar to this:
http://cgi.ebay.com/PW-200-M-200W-D...QcmdZViewItemQQptZPCA_UPS?hash=item1c101bc8ec
So a 300-400w version is out of the question. Even if it does "burn out" after a while it's not that expensive. I'd probably add a seperate 12v > 5v PS for USB and HD use.