The usage of electricity of a desk top is very low . In the region of a few watts per hour - in the 30s. If you have it on the energy saving settings the screen will power off and ot will go to standby when not being used. There is more strain on the compueter when switching it off and on so it makes sense to leave it on when in use but ready to go. If it used 50 watts per hour then in 20 hours you use a unit and you will know what you pay for that - I pay 11.3p in the day inc VAT. So if you had it on from 8 am to midnight you use , say, 16 x 50 = 800 watts and that would cost me 0.8 x11.3= 9p. So per week = 63p , per year £32.85 assuming you have it on Christmas Day as well. of course electricity will have gone up in price by then.
So if you now switch it off for half that time you could save money! But tecomputer helpd heat your room ( a tiny amount) and teh power box does wear out , but usually the hard drive will pack up first - or the chip or the heat sink clloer on the chip. I have never had a power drive fail - yet but hae had a chip cook when cooler failed.