On Sun, Feb 07, 2010 at 10:10:22PM -0500, Nick Holland wrote:
quoted text
> With all this talk about power reduction...I'm going to toss out one
> small suggestion:
> 
> Get a Wattmeter, and measure...  Don't waste your time speculating.
> 
> An ammeter and high school physics V*A=>Watts doesn't cut it for AC
> (in general -- a lot of machines are power-factor corrected now so V*A
> becomes QUITE useful again, but some have a really big power factor
> still...just discovered a P4-vintage machine running a power factor
> of 0.65, which surprised the heck out of me.  And if you have no idea
> what I'm talking about, just get a good Wattmeter that understands
> real AC Wattage, and don't worry about it).
 
Even though the cheap ones try to measure real power and not apparent
power, they are often very inaccurate especially at low loads. Watch out
for cheap meters, and if you have multiple similar machines it's not a
bad idea to measure them all at once.

If you don't believe me, buy two cheap meters of the same kind and one of
another kind, if they're within 10% of each other (with a non-resistive
load and low power) I suggest you go buy a lottery ticket. They're still
useful for relative comparisons, of course.

With a machine consuming 100 Watts, switching from a 65% efficient
supply (that's an optimistic guess for a cheap power supply a few years
ago) to a 80% efficient supply will save 28 watts. If you live in a hot
climate and use air conditioning, it's probably worth it in a 24/7
machine. For a machine consuming 50W the savings are probably not worth
the investment. If you are buying a new machine, the price difference
between a crappy and a good power supply is so small it's a no-brainer.
As a bonus the high efficiency supplies run cool and quiet.