There's not much that intrinsically makes an aarch chip lower-power or more energy-efficient than an x86 or x64 one – energy use will be more about the design

There's not much that intrinsically makes an aarch chip lower-power or more energy-efficient than an x86 or x64 one – energy use will be more about the design goals of the microarchitecture than anything else. There's a paper from a few years ago the looked into this in some detail – ftp://doc.nit.ac.ir/cee/computer/alinejhad.saeedeh/old/95-96-1/MS/Advanced%20Architecture/project/paper/6-ISA%20Wars%20Understanding%20the%20Relevance%20of%20ISA%20being%20RISC%20or%20CISC.pdf – there's no immediate reason to think the conclusions should have changed.