Michael Semcheski wrote:
On Fri, Aug 22, 2008 at 1:40 PM, Christopher Snook <csnook@xxxxxxxxxx> 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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