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?

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)?