In message , at
06:38:05 on Tue, 17 Mar 2015, whisky-dave
remarked:

Working harder when the scene being shown was shot at night, less hard
when there's lots of white in it.

and what exactly happens when the screen works 'harder' ?

The backlight gets brighter and as a result they can claim a better
dynamic contrast ratio because scenes which would otherwise look like a
sea of mud have some detail visible in them.

another vairiable, burning a DVD which I know takes an extra 5W of
power
which obviously all comes from the household's total consumption,

Yes, but again, much more slowly than TV scene changes.

Irrelivant.

That is how you work out what you are seeing current
wise is due to.

so what is this current ?

Driving the backlight.

Yes I want to know what magnitude this current is or what value it is.
without knowing this you won't get anywhere.

so what current does a backlight use in a TV .

I posted some numbers a few days ago, measured from my clamp-on device
at the electricity meter (which transmits the result every 4 second to a
little box I have on my desk). Somewhere in the range 30-40 watts out of
about 100 for the TV as a whole.

--
Roland Perry