Estimating System wattage

[Christopher Snook]

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.)