How much does a watt cost?
How much will a 1-watt device cost you if you leave it running 24x7?
I once heard a very useful rule of thumb:
A 1W device costs £1/year, if it’s running continuously..
At the time, this was almost exactly right for most people in the UK, and it made it easy to calculate the effect of, say, using that old 200W PC as a server for the next three years rather than getting a more modern one.
Sadly, for most of us, energy costs are going up, so you might want to do the calculation for yourself.