Fwd: OT: Energy efficiency difference btw GNU/Linux, Mac & MS
I think your question might be a category error.
The power consumption of a PC can be broken down component by component. A PC with hardware that's been spec'ed with low energy consumption in mind (e.g. the XO-1 from the One Laptop Per Child project, which draws ~2W in typical use) will consume far less than a typical desktop in normal use (~200W, at a guess). Each PC will have a given maximum power draw: the amount drawn when all HDDs are in use, the processor and graphics card is being maxed out, power-hungry bus-powered peripherals are connected to the USB ports, etc.
The operating system (and, in some cases, the BIOS) can have an effect on the conditions under which other software on the machine can put it in a state in which it is drawing maximum power. This is power management. All major operating systems have power management capabilities. The optimisation of these capabilities depends far less on the underlying operating system than it does on the machine's administrator's understanding of the machine's user's work habits.
Finally, inefficient code will require more processor cycles than efficient code (and maybe more HDD hits, etc). So efficient code will also be more energy efficient. Here, perhaps, there is room for answering your question. But I expect that the efficiency of operating system code may vary according to the application(s) running. So quantifying this will be very difficult (not least because MacOS and Windows are closed-source, so the efficiency of their code could only be determined experimentally. Also, compared to the other factors mentioned above, differences in the efficiency of operating system code are likely to have a negligible effect on power consumption, except possibly in edge cases.