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.
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.