In article , David Hearn dave@NOs
wampieSPAM.org.uk writes
We have a 300W halogen up-lighter with built in dimmer. It also has a
50W bendy spotlight with dimmer.

I'm aware that 300W is a lot of electricity, and in fact, it's too
bright when on full - we tend to have it on 50% or less.

Does dimming a light reduce the electricity usage? Part of me says yes,
but another part of me is less sure.

I'm thinking of looking for a 150W lamp to go in there which would be
more reasonable - especially if dimming has little effect on electricity
consumption.

I know that the efficiency of incandescent lamps falls off rapidly with
decreasing drive, particularly for halogen lamps but I didn't want to just say
'it's ****' without an authoritative reference.

Not much, or should I say too much on google, the closest I could find in a
quick search was an efficiency study of halogen torch bulbs by a caver:
http://www.cs.indiana.edu/~willie/lvr.html#anchor00021

If the theory scales up it suggests the following:

300W at full brightness costs 300W (100% _relative_ efficiency)
Dimmed to half brightness costs 230W (65% rel eff)
Dimmed to quarter brightness costs 170W (65% rel eff)

ie. your savings are not great.

Your 150W bulb will hopefully give you the equiv brightness the half
dimmed case but for 80W less.

150W dimmed to half bright costs 115W (65% rel eff) and should be equiv
to the 300W 1/4 dimmed case but at 55W less useage.

HTH
--
fred
Plusnet - I hope you like vanilla