"Andrew Gabriel" wrote in message
...
In article ,
Andy Dee writes:
Trevor Smith wrote:
Today I recieved my British Gas electricity monitor and I have noticed
that when I turn on the kitchen lights that are two 58 watt T8 tubes
running on two conventional ballast gear trays the wattage on the
monitor shows a draw of 160 watts which means that if the tube draws 58w
then the ballast is drawing 22watts, would this be correct?.

Trevor Smith
Just be aware that any device that only measures current cannot be an
accurate watt-meter.

Yes, it can't take the power factor into account.
Your fitting must have a power factor correction capacitor, or for a
a 58W tube, it would register about 2.2 times the actual power used.
A capacitor can't fully compensate though, because part of the low
power factor is not simply due to phase shift, and the capacitor can
only compensate for the phase shift component.

Ballast efficiency is marked with an EEI (Energy Efficiency Index)
rating on the ballast.

With the information you've given, I can't tell how much of the 22W
is a misreading due to power factor, and how much is ballast losses.
It can't possibly all be ballast losses, because the ballast would
overheat. For a 58W tube and ballast, max ballast losses a
EEI=D 12W
EEI=C 12W
EEI=B2 9W
EEI=B1 6W

If you are interested, you could pop the cover off, and read what
the ballast EEI rating is. It's no longer legal to sell EEI=C or D
ballasts in the EU.

If you want a more efficient light, rewire the luminaire with an
electronic ballast. You can get one which will do twin tubes
(although both will go out when one dies), or separate ballasts
for each tube. Electronic ballasts are required to be more efficient
than even the EEI=B1 rating.

--
Andrew Gabriel
[email address is not usable -- followup in the newsgroup]

Just had a look and the ballasts are EEI=B2 and whilst I was doing that I
check again the load on the monitor and it now shows that the two lights are
drawing 120 watts which means with the EEI=B2 ballast losses taken into
account they are more efficient than they should be.
Conclusion, these monitors are a very rough guide and I will properly do
what PeterC did and give it away.

Trevor Smith