Estimating System wattage
Christopher Snook
csnook at redhat.com
Fri Aug 22 21:11:11 UTC 2008
Michael Semcheski wrote:
> On Fri, Aug 22, 2008 at 1:40 PM, Christopher Snook <csnook at redhat.com> wrote:
>> You only need massive power supplies in a desktop if you're running high-end
>> video cards or something else that needs separate power inputs. Enjoy your
>> very long UPS runtime.
>
> In my experience, peak power usage can be much higher than the average
> power usage.
>
> e.g., at boot time the system uses lots of power to spin up the drives
> and fans. Once their rotational velocity is established, power usage
> can drop to 50% of peak and stay down.
Yes. This is why I have a 350W power supply on my desktop that peaks at 117W
under sustained load. 750W is pushing it. A lot of that power is going to be
reserved for PCIe rails, which you're not using unless you have high-end video
cards, and it's no help during spin-up. You'll bleed a few watts on those rails
just keeping them hot, since the power supply has no way of knowing whether the
lead is dangling in the air or hooked up to a device that happens to not be
drawing power at the moment.
> The other line of reasoning I've heard for buying an over-sized power
> supply is because running the power supply closer to capacity will
> shorten its life. Also, power supplies may be more efficient running
> at a fraction of their maximum rated capacity. i.e., a 200 watt power
> supply delivering 150 watts of power is less efficient than a 450 watt
> power supply delivering 150 watts of power. Maybe someone who really
> knows can respond to this point.
You're very right about the lifespan when the power source is unclean. If you
have a good UPS, this is much less of an issue.
As for efficiency, the peak is usually somewhere in the middle. 80+
certification tests at 20%, 50%, and 100%, and the enhanced certifications have
stricter requirements at 50% than at the extremes.[1] I'm sure there's more
variability among low-end supplies, but if you've got a 350W and a 750W power
supply of equal quality, and both are drawing 120W from the wall, the 350W
supply running at 34% load is probably generating a lot less heat than the 750W
power supply at 16% load.
-- Chris
[1] http://www.80plus.org/manu/psu/psu_join.aspx
(Mouse-over for testing requirements.)
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