Estimating System wattage

Mike McCarty Mike.McCarty at sbcglobal.net
Mon Aug 25 19:12:37 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.

This is correct.

> 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

This may or may not be correct. What kills most components like that
is heat. If you conduct the heat away, the PS will last a long time.
Running at reduced power helps by not generating as much heat. Sort of.

> 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

"May" is definitely the watchword here. SMPSs like a minimum load. Below
that, they lose regulation, and some will explode. So, many PSs include
a minimum load inside them. This minimum load is absolute waste of
energy. Running an SMPS at very light load results in low efficiency.
I'm not an expert with UPSs, but I'm sure they use some sort of SMPS
to generate a sort of "stepped sine wave", and suffer from some of
the same issues. Anyway, using an ordinary PC power supply which
is way overrated can result in low efficiency.

> 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.
> 
> Mike
> 


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