From my personal experience with my tablet, and from the benchmarks and articles I've read, it always seems ARM processors, as seen in virtually all mobile devices, deliver incredible performance for the amount of power they consume. It really seems like ARM is inherently more power-efficient than x86.
To me, it looks like that this is because x86 is a CISC architecture while ARM is a RISC architecture. However, Intel Atom processors deliver very good performance relative to their power consumption as well. In addition, having studied computer architecture, it is my understanding that in modern processors, the complex x86 instruction set architecture (ISA) is internally translated into simpler micro-operations that use an internal instruction set that is closer to the simpler ARM or MIPS ISAs than x86 proper. As such, the ISA as presented to the programmer is little more than an interface to issue commands to the processor, rather than a representation of the actual low-level operations the chip performs. Then again, implementing this translation layer requires additional silicon space on the chip...
That said, assuming that they are implemented using the same semiconductor process, is ARM inherently more efficient than x86? With a properly designed microarchitecture, is it possible for an x86 processor to deliver the same performance per watt as an ARM processor?
Remember, I'm looking for a well-written technical answer in the spirit of Stack Exchange and not mere speculation.