I’m not sure if this is what you were implying, but I don’t know of any x86 processors that can compete with the Arm processors that are in use, on power consumption to performance ratio. Take e.g. Apple’s A12, which compete with their MacBooks in performance, and assuredly draw much less power.
You haven't been paying attention. In order to go faster ARM started using more power. A lot more power.
Turns out power usage was never an ARM vs. x86 thing, it was purely a "how fast do you want to go" thing. ARM started at the "very slow" end of the spectrum which made it a good fit for mobile initially since x86 didn't have anything on the "very slow" end of things. By being very slow it was very low power. But then the push to make ARM fast happened, and now ARM is every bit as power hungry as x86 at comparable performance levels.
The power cost is for performance. The actual instruction set is a rounding error.
> I don’t know of any x86 processors that can compete with the Arm processors that are in use, on power consumption to performance ratio
Not anymore, but there was a time when x86 was (barely) able to compete in that area and there were some x86-based smartphones and tablets. But it was too little too late: x86 already was a niche. Developers absolute had to support ARM, but x86 was optional, so many apps were not available for x86, and that was pretty much it for those devices.
