The more expensive watches (Fenix) also have long battery lives: lasting up to a month on a battery that can fit in a watch. The processors still have to sip power.

The Coros watches are less than half the price, have 22 days of battery life in smartwatch mode (in the Nomad) and render maps extremely fast. If they added solar, they could probably also last a month (the Coros Apex 4 does 24 days, also without solar).

The funniest thing is that earlier versions of the Coros even used the Garmin map format (though as many small files and not a single/small number of .img). Though they have switched to the open PMTiles format in later versions.

BTW, I had a Fenix 7x solar (before a Fenix 8 AMOLED) and it would usually 'only' last about two weeks. I think you can only reach Garmin's stated time if you disable a lot of functionality.