Just to throw this in as people are talking about bills etc.
I don't live with parents, and pay my own bills. But from my experience switching the PC off when not in use only saves me about £5 a month. I'd continue to switch it off when not in use if I was saving about £12+ a month, but this isn't this case for me.
I do have a watt meter, and the PC, while on a game, takes about 600 - 700W, on the desktop it uses about 300W, if I can remember right.
Then something must be awry with your sums or you are on your machine almost all day every day.
At 15p per kwh 100w costs you 1.5p per hour.
Thats 36p per day
and thats £131 per year just for 100w.
If you turned off at the wall for 8 hours a day (not an unreasonable amount considering sleep and work) then you'd save 1/3 of that cost. So £43 a year.
The only other thing I can think of is that when the machine is off you are using something else more which is sapping the juice.