If you are talking about current power consumption of different parts of PC - no such software exist.
If about whole PC - max = PSU max
or am I missing something..
Well I've read that all software is dependent on what info the motherboard/components will feed to it. So you may be right no solution exists.
There was one guy who I saw said he had a meter on his UPS that he used to see how much voltage was being used. That sounds like some solution. Perhaps the only solution would be to have a piece of hardware between the computer and the wall socket.