Questions about UPS+surge protector

James Knott james.knott-bJEeYj9oJeDQT0dZR+AlfA at public.gmane.org
Tue May 18 01:05:53 UTC 2004


Terrence Enger wrote:
> At 09:59 2004-05-15 +0300, Peter L. Peres wrote:

>>As a simple rule of thumb, add the nameplate ratings of the monitor and
>>computer (e.g. 500W PSU + 100W monitor = 600W) and get a UPS with similar
>>nameplate rating (600VA). this is not entirely correct but it should be
>>close enough for your purposes (the VAs are Ws corrected for the power
>>factor, aka caos phi. VAs are also known as real power. 
> 
> 
> Are you sure you have this the right way around?  I thought VA was the 
> unit for reactive power,

VA (volt amps) refers to the current and voltage in a circuit.  Without 
knowing the phase angle, you can't determine power.  In a purely 
resistive circuit, the phase angle theta is 0 and the VA = power.  In a 
purely reactive circuit, theta is 90 degrees and the power = 0 W.  To 
properly determine the actual power used, you need to know the voltage, 
current and phase angle, such that power is VA cos theta.


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