But really, is 40W or thereabouts worth worrying about when
your PC is eating a couple of hundred?

Don't be fooled. The 200W PC power supply rating is just like your consumer
unit being rated at 100A. It doesn't mean it is actually eating it all the
time. Whilst idling, a modern PC uses a tiny fraction of this. Even less if
it turns off the hard disks after a period of inactivity.

Many of the components of a desktop PC are similar to those of a laptop.
That runs for hours off a little battery. OK, the desktop versions are
usually a more power hungry, but not by that much.

Christian.