I'm pretty sure that x86 could be more power efficient than it is now. But I have rarely run into an implementation of x86 that wasn't pretty dang power hungry. Probably for the explicit reason that it isn't a huge priority for the stuff they run it on.

I think the lowest power ones are Atom and Celeron. All the other low wattage CPUs are RISC based. I think AMD is also coming up with something similar.

Nearly all x86 CPU architectures originate from desktop/server CPUs which had much higher thermal and power limits, but nearly all ARM CPU architectures originate from embedded CPUs where there are strict thermal and power limits.

It's not as if there's something fundamental with x86 that makes it use more power. There used to be the case that x86 CPUs had many more transistors (and thus more power usage and heat) due to the need to implement more instructions, but I don't believe this is the case anymore. I think right now is that its easier to scale up an architecture, than it is to scale down.

If you look at something like Project Denver, it started as a x86 compatible CPU that would of had ARM-like power usage.

> I think the lowest power ones are Atom and Celeron. All the other low wattage CPUs are RISC based. I think AMD is also coming up with something similar.

AMD Had Bobcat and later Puma, IIRC there were some <5W parts in both of those families. I still wonder -why- they never did a dual channel version of those parts for desktop, my guess is it would have made Bulldozer look extra bad somehow.