Short answer: it dependsProbably a dumb as hell question, but I don't recall anybody asking it sooooo... (I'm sure it came up at some point, but anyways) With all of the talk of BC (improving base switch games dyn. res. and framerates) are battery gains a thing we can expect from last-gen titles played on the succ? Or it doesn't work like that?
Longer answer: Historically BC modes have frequently involved downclocking and turning some hardware off, so there would generally be clear gains in battery life, but that was with a primarily hardware driven approach. With software driven BC, there are certain knobs available to Nintendo which can and probably will lead to some minor power savings in some cases, but probably not across the board. Lowering clocks is something that may be done for compatibility, but probably not for every game. Similarly, parking unused cores could help save some power, but only if the system doesn't need those for emulation.