Wouldn’t that seriously hurt laptop battery life to use a discrete gpu to render the UI? Or is there someway to tell the OS that a non-discrete gpu is fine?
No, GPUs are more power efficient than CPUs to render graphics, so it'll save battery.
So, my laptop is dumb for turning off the discrete GPU when I unplug it?
They didn't say discrete GPU, they said GPUs, which includes integrated GPUs.
A game engine will tell the OS to start up the discrete GPU if it is available. This is why I asked the original question. Is there some way to turn that off?
When initializing a GPU context you can specify whether you want the most efficient device, the fastest device, etc, for a GUI app then you would want to pick the most efficient, which is a single flag to pass in, or supply options and let the user pick which one they want.