There's no such thing as "100% efficiency conversion to heat".
Why not? Of course there is! Where does all the electric energy go that your PC consumes? I'll split it out for you:
- Most part is converted into heat directly in wire resistance and switching losses of transistors (in power supply, processor, logic, etc).
- Then there's light from the LED's and your monitor. But light ends up being transformed into heat ALWAYS, whether it is inside the room you're in, or outside (that portion of light that escapes through your windows).
- Then there's acoustic energy, from vibrating hdd heads, fans, etc... but that also ends up as heat being absorbed by walls and whatever (except maybe for the tiny fraction of noise that escapes the room).
- And finally there's moving air (from the fans) which ends up converted into heat through friction.
Conclusion: your PC is as efficient as an electric heater in generating heat... difference is that your PC can do something useful (other than generating heat) in the meantime (like downloading pr0n
Please note that I never intended to say that it is a good idea to leave your PC powered-on when not used. I merely wanted to make a point that figures stating the supposed amount of wasted energy are not quite realistic.... but it still is a waste of energy.