Devices (especially OLED devices) do offer their own native battery saver mode, this must be set up in your watches settings. This does not have to do anything with the battery saver mode integrated in my watchfaces. During the time this battery saver is active a custom watchface is fully disabled by the watch itself.
This one does only have an influence if the native battery saver is not enabled or at least is not active currently. In this case I do offer a custom minimal watchface (that is a little different from the native one) to minimize my watchfaces battery impact even if you don’t use the native battery saver mode. Of course I do have full control over what I show and what not in this case and therefore I avoid all “heavier” operations and only show a minimal layout. Additionally I can control if I disable this battery saver mode during a gesture e.g.
Custom watchfaces always need more battery then a native one so if you want to save as much battery as possible, enable the native battery saver. If you want to optimise battery usage inside my watchface (this is the time where the native battery saver is not active) you get additional possibilities with the build in battery saver inside my watchfaces to tweak battery usage when my watchface is active even further.