You could start running things on ARM, but, almost certainly, that comes with a lot of extra friction. (Not saying that isn't a bad idea, it'd probably improve the ecosystem as a whole and flush out architecture-specific assumptions in server software. But it's not someting trivial to do.)
If the processor is mostly idle or running minimally optimized software, which is most software, then ARM offers better performance per watt. If the processor is running highly optimized code at max throughput all the time then x86 offers better performance per watt.
This is an intrinsic tradeoff. To make low-utilization workloads more power efficient you have to make high-utilization workloads less power efficient and vice versa. ARM and x86 differentiate themselves by taking opposite ends of that tradeoff spectrum.
It depends on the code.
x86/64 is looking more and more like the next Alpha or MIPS in many ways.