Here’s a very rough rule of thumb which I find exceedingly useful:
If something uses 1W of electricity, and it’s switched on all the time (24 x 7), it will cost about £1/year in electricity.
So, for example, I have an elderly Mac Mini next to my television which used to be on all the time because it was my ‘media centre’ – it recorded things from the TV onto disk, etc. It takes about 80-100W, so it costs me roughly that many pounds per year, which means that if I turn it off I can get my Netflix subscription for free!
All sorts of calculations become pretty straightforward.
I have a second display for my iMac, which uses about 60W. (I’ve just measured it.) So that would be £60/year, but it’s only on for about 8hrs/day, so £20/year.
If a salesman tells you a new fridge will save you an average of 40W compared to your current fridge, but costs £400, you can work out easily that it’ll pay for itself in about 10 years.
In case you’re curious, this rule assumes you pay about 11.5p/kWh for your electricity, which is close enough for most UK residents. I forget who first pointed the convenient annual multiplication out to me, but I find myself using it all the time, so I thought I’d pass it on!