"Scott" wrote in message
...
I was watching an item on TV about use of batteries to balance the
national grid. I have also heard it said that the supply is likely to
become less reliable with increased dependence on renewables.

I have always assumed that an uninterruptible power supply must be
energy inefficient for the same reasons as stand-by uses electricity.
Is this a correct assumption or does a UPS not consume any electricity
at all unless it is brought into use?

Depends on the type of UPS. The best of them charge the batteries all
the time and use the batteries to power what it is powering, so all that
happens on a mains failure is the batteries stop being charged. Those
clearly do waste some power all the time. The other sort that switch over
on a mains failure only waste power when replacing the self discharge.

As an aside, could a power cut ever damage a modern computer (say if
you happened to be installing an update or defragmenting a disc when
it happened)?

Not when defragging. It can happen when flashing the bios.

With updates, it depends on whether you have a full backup etc.