I heard that apparently the M-series chips have some dedicated hardware for emulation, I may be wrong however. Not sure though how it's powered, and how it does the emulation itself afterwards. It very well could improve battery life if it does more than blind emulation. For the opposite end, look at QEMU (kind of an unfair comparison since Rosetta seems to be more like an userspace emulator, whereas QEMU handles a ton of devices and more)