https://wccftech.com/dark-mode-on-oled-iphones-will-give-you-30-extra-battery-life-than-light-mode/

A new automated battery life test has shown that Dark Mode in iOS 13 can provide approximately 30% more battery life than Light Mode, on iPhones with OLED displays. The test was done using apps that support iOS 13's native dark mode, as well as Google's apps, which do not support a pure black theme, but a dark gray one.

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