The “Kill-o-watt” is the only way to really tell since computers vary widely.
The CPU can draw up to 135W or under 3W on it’s own under full use, depending on which one you have. Some computers have multiple graphics cards that draw almost 300W each while some make do with integrated graphics that draw next to nothing. And how many hard drives does it have? A RAID array (multiple drives acting like one) draws more power than a single drive.
And then there are monitors. Old-school CRT monitors are energy hogs, especially if you have a 20” like I used to before switching to a flat-panel LCD.
So without knowing more about the computer, it really is impossible to say. I have one that draws 14.7W and another that can draw over 400W, and that one isn’t even all that high-powered like most gaming rigs. Either give us a complete parts list, or just do the easy thing and try a Kill-o-watt (or similar) and measure it directly.
I will say that my roommate leaves his computer on 24/7 and the other desktop is on at least six hours a day and our total electric bill (including electric heat and the electric stove) hasn’t hit $140/month yet even in the winter and averaged closer to $75.