[OT] Re: Leaving a server on all day

[Charles Swiger]

On Jun 8, 2004, at 5:06 PM, Bernt. H wrote:
>> No need to guess, use an amp-meter.  :-)
>
> Well If it measure trueRMS then you could use it, otherwise no.

You are correct that one needs to measure the voltage and use the RMS 
value, or DC series equivalent if you like that phrase, in order to 
figure out the power consumption accurately, but an {ammeter, 
amp-meter, DMM} which can deal with AC will do the right thing.

>> Radio Shack and the like will sell something with male and female 
>> plugs that will measure both voltage and current, and give you the 
>> current power load in Watts.  Smart UPSes may also have a similar 
>> capability.
>
> Yes but it will only show you the correct value if the load is a pure 
> resistans, not if it's reactiv, as all switching psu's are.

The ratio between the actual load and a purely resistive load is known 
as the power factor, and is why UPS are rated in terms of kVA rather 
than in terms of the wattage of the load.  For computer equipment [1], 
the power factor is lagging, representing an inductive rather than 
capacitive load, and the PF is typically about 0.9.

However, the electric company bills you for the power you draw from 
them, they don't give you a refund for the power "wasted" because your 
load is not purely resistive, so the notion of measuring the kVA rather 
than the "useful wattage" is not really "incorrect".

-- 
-Chuck

[1]: And almost everything else, too.  Most things use a transformer to 
convert line voltage into whatever voltage the device wants, which is 
inductive, or consist of a motor, also inductive.  Motors which draw a 
lot of current when starting (which is most of them) tend to have a 
"starting capacitor" to help manage the surge current and also help 
adjust the power factor back towards 1.0 to improve their efficiency.  
The so-called "ballast" in fluorescent lights serves much the same 
purpose.

We thank you for tuning in to basic electronics, and return you to your 
regularly scheduled FreeBSD programming.  :-)