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.
alt Hacker News
The reality is more complicated than this.
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.