Both theoretically and anecdotally, we know that dark mode makes a difference to phone batteries. Apps that change their default background hue to black rather than white just seem to last longer, and that logically makes sense – on OLED phones at least.

Now we have proof in the theoretical pudding. YouTube channel PhoneBuff has conducted a pretty conclusive test using two brand new iPhone XS Max handsets. Both handsets were set to 200cd/m2 brightness, and then robots ran through automated tests using four apps with dark modes.
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https://www.trustedreviews.com/news/yes-dark-mode-really-can-save-phones-battery-3948266