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I switch my PC and monitor off at the wall while they're not in use. I was sent one of those energy meters when I changed electricity suppier and went around the house checking standby power.

Depends. Mine uses ~150 W when idle (with monitor off), which at 16p per kWh would be 2.4p per hour or £17.28 per month.
I turn it off when not being used ever since I got a wattage meter!
Before that it was on first thing in the morning and off last thing at night!
turned off. With SSD, doesn't take long to start up...
Server stays on 24/7 and uses a fair bit of power, but I can't really do without it.
I switch my PC and monitor off at the wall while they're not in use.