Okay, a bit of theory here : any screen uses lightsource to display images. So brightness means that yhe lights are brighter so they use more energy. HOWEVER! Using Dark themes will help you A LOT! Why? A "black pixel" is actually "not a pixel" but a light turned off in the screen. Don't freak out, it's normal and it doesn't mean your screen is broken if you are using black/dark theme and your screen have a few pixel turned on.
Remember :
The darker a pixel is, the less power the light uses to make this color appearing.
Black = off
Grey = 50%
White = 100%
Personal experiences with android devices that might help you : if you have an auto-brightness setting, DO NOT USE IT. Why? It constantly analyzes the surrounding light amount and adjusts on it's own.
Instead :
When you are playing with handheld mode, turn on Powersaving and reduce brightness up to 20% or 30%, if you can view better, try out 10% or 15%, don't go lower because you're going to force your eyes.
Also, I might be wrong on that point, but as soon as you have to do something like "going to eat lunch", put your Switch in Sleep/Standby mode instead of turning it off.
Talking about energy saving. I started a new technique that helps a lot! If you are using AAA, AA, D or C batteries...
Remove them from the controller you're not using... I know I know... "Why... just why?"
Be aware of that : when batteries are left in a controller, even if the controller is turned off, a very tiny bit of juice is used. By removing the batteries, you literally just "cut" this "juice draw".