Mail Archives: opendos/2001/04/09/17:49:02
>Wow! For an O/T, this stuff about SI/Imperial/American terminology
>and measurements sure generated a lot of messages!
>
>Anyway, back to the original topic ...
>
>You cannot determine the power consumption of PC's and monitors
>by just measuring the current - even if you measure this in RMS! Of
>course, if the current measurement isn't RMS, then that's even more
>useless!
>
>If you measure the RMS current and multiply by the (RMS) voltage,
>what you have is the "apparent power" (VA), not the "real power"
>(W). That's the reason why the "power" figures quoted below are
>higher than expected. Now, "apparent power" (VA) is the _vector_
>sum of "real power" (W) and "reactive power" (VAR).
>
>So, what is "reactive power" (VAR), and what impact does it have?
>Well, it's the out-of-phase current ... it is current that recirculates in
>the power system but, apart from resistive losses in the wiring and
>generation system (ie. second-order effects), does not require any
>input (eg. coal) to the system. If we used superconductors for all
>this stuff, then even these resistive losses would disappear!
>
>So, the only way to correctly measure the power consumption of
>PC equipment, is to either use a proper power meter (hard to find)
>or use an energy meter (eg. the one at the front or side of your
>house) and see how much energy it accumulates in one hour (if
>necessary, run the test for several hours, then divide the energy
>accumulated, by the number of hours).
It is true that the "reactive power" is not real power, but only at the
consumer end of the line. From the other side (where it is generated),
this makes more energy to be lost in the distribution wires. What
happens then is that if you have too much "reactive power" you will
be billed extra (something like a fine). On the other hand, PC supplies
have a shift to the capacitive side which can somehow compensate
for the inductive power used in most industries and contribute to
fewer losses. I believe only a macroscopic view of the problem could
get close to reality and know of no study about this.
One good way of measuring power consumption is by the bill you
receive (a philiphical point of view), another one is the ecological
impact. I believe the later is better and by this standard we should
turn everything off when not in use ;-)
Alain
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