[Gllug] Energy consumption puzzlement

[James Roberts]

M.Blackmore wrote:

> Turning OFF the printer (and hp 850c deskjet) INCREASES the electricity
> consumption by 3-5 watts (varies cyclically over a couple of seconds).
> 
> I'm surprised at this...
> 
> BUT why should turning it off INCREASE the consumption?

First, don't believe the power meter for resolution of better than a few 
percent (unless it's a very fancy one - and then still don't believe it)

Second, try checking the power factor figure with the other equipment on 
and off.

It is conceivable that the other equipment is improving the power factor 
  when it is on. This would lead to an apparent reduction in consumption 
(depending on the was the meter is built and calibrated).

These meters are good for getting a general idea. However the precise 
measurement of true ac power is complicated. Most of us are more 
interested in cost and power factor is implicated in that (in the UK) 
which is why pf correction is fitted to fluorescent ballasts in the UK 
(but not in Europe).

Of course it might not be that... another problem is the switch-mode 
psu's universally employed which draw a very spiky current, extremely 
difficult to accurately integrate for rms values. If the other gear is 
using a different part of the mains waveform then this could reduce the 
peakiness and hence the apparent measurement.

You can however be confident that less power is drawn when it is 
unplugged :)

JR
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