A desktop is an always-available load if you never shut it down. A modest office tower might average 50–100 watts while you work, less in sleep, more if it has a gaming GPU. The monthly number is that average times the hours it is actually awake.
Sleep/hibernate is the cheap habit. A full tower left on overnight for a download you forgot is hours of the higher draw. A NAS or always-on home server is a different card if it never sleeps.
The monitor is separate. Two screens can match the PC.
Set awake watts and hours. If the machine sleeps 16 hours a day, do not price 24 hours at full load.
At 18.8¢ per kWh, 80 watts for 10 awake hours is 0.8 kWh — about 0.15 a day. The same PC left on 24 hours is about 0.36. A gaming session at 300 watts for 3 hours is about 0.17 on top.
Sleep
Do not price 24 hours at full load if the box sleeps 16. A NAS that never sleeps is a different always-on habit.
Related
Usage estimate only — not a utility bill, load calc, or advice.