I have found that when I have an electronic viewfinder, I don't use the screen. On my Powershot G5X with a rotating screen, I kept the screen permanently closed, using only the electronic viewfinder. I imagine not using the screen should increase battery life. However, if I require a rotating screen that strongly limits the available models for my next camera. Is it true that using only the electronic viewfinder, with the screen switched off, substantially (>20%) increases battery life?
I have specifically mirrorless cameras in mind. That may mean the answer is different than for DSLR.
(If yes, I will next ask a question whether there is a way to keep the screen permanently switched off on models without a rotating screen.)