Background Paragraph: In Windows 2000 and later the threads in the System Idle Process are also used to implement CPU power saving. The exact power saving scheme depends on the operating system version and on the hardware and firmware capabilities of the system in question. For instance, on x86 processors under Windows 2000, the idle thread will run a loop of halt instructions, which causes the CPU to turn off many internal components until an interrupt request arrives. Later versions of Windows implement more complex CPU power saving methods. On these systems the idle thread will call routines in the Hardware Abstraction Layer to reduce CPU clock speed or to implement other power-saving mechanisms. 
Story: Bob is building a new computer and wants it to be the most advanced version he can make. He has all of his parts, but is not deciding on a operating system. He sees that Windows 2000 is on sale, but also isn't sure if it will do what he wants, and has the choice to pay a higher price for Windows 10. His other goal is to make sure that his machine is efficient, and won't cost a lot in the way of power costs while running, since he leaves his machine idle most days.
Which operating system should Bob avoid if he wishes to have high power efficiency? Which operating system should Bob buy if he wants the best power efficiency? Will Windows 10 be more or less efficient than Windows 2000 in power saving? Will Windows 2000 be more or less efficient than Windows 10 in power saving?