[OT] VA=W?

["Paul Robinson" <ukcueman@xxxxxxx>]

This has been bugging me a little. It's probably trivially simple to the
electronics people here. I didn't get a satisfactory answer from an
electrician I know...maybe somebody can enlighten me.

Transformers (eg for halogen lights) and uninterruptible power supplies are
both rated in terms of VA.

So what is the difference between VA and watts, because I'd always assumed
they were the same. Obviously, they are both a measure of power. I guess
the
answer lies in the fact that I'm talking about AC and the voltage is
expressed as an RMS term or something like that.

So why are transformers rated in terms of VA instead of W? And is there any
difference?

Can you work out the current drawn by many lights by adding up their power
rating and dividing by 240? Or is it more complicated than that? If some of
the lights are low voltage halogens, does the calculation change in any
way?

My sparky friend said something to the effect that a 50W 12V halogen
actually draws less than 50W on the 240V side. This seems counter-intuitive
to my primitive knowledge of electricity since I would have thought the
power used on the low voltage side would have to equal the power used on
the
240V side plus the power given off as heat (and sound) by the transformer.

Can somebody put me out of my misery please?

Thanks,
Paul