The M1 going full tilt will use 35+ watts, vs like the A15 which I think people guess is limited to around 7 watts. I have a 10th gen and it gets outstanding battery life doing basically anything. Meanwhile the M1 powered iPads can actually drain pretty quickly. The CPU is efficient compared to a core i9, but 30 watts in a tablet is a lot for their battery.
But the latest phone CPU constrained to say sub 10 watts would last an insane amount of time on even the smallest battery you'd put in a tablet/keyboard glued to a tablet running Mac OS. I get 10-15 hours of battery life on my M1 MBP, and most of the time I don't really need all that performance. If I could double that that would be insane.