Running Linux on a laptop is a bad idea - always shortens your battery life.
This is mostly because of drivers - the linux drivers for things like video cards are not as good as the Windows drivers, and so your battery life is shorter. Its a well known problem, google it.
I also doubt that something Ubuntu is lighter in resources than Windows, especially not when using the legacy X11 windowing manager. Some Linux Mint flavours are quite lightweight.
I am okay with my battery life being shorter than what it should be. I seldom use my laptop off plug if I am working. If I do, max 3 hours, of which I have 3 hours of battery life left.
Ubuntu offers Wayland as its replacement to Xorg. I have been using that for a while, a major improvement over Xorg.