It was only a couple of weeks ago that Dark Mode for iOS and Android devices was rolled out, and since then users have been wondering if using Dark Mode affects the devices battery life. Tests have been released and it seems that it does.
Tests conducted by PhoneBuff indicate that for iPhone users who have upgraded to iOS 13 and have enabled Dark Mode will be given 30% more battery life. The set of tests conducted by PhoneBuff were with robots who did a bunch of predefined tasks that included typing out messages, scrolling Twitter, using YouTube and Google Maps. The phones that were used for these tests were two iPhone XS Max's that were brought to the same level of brightness to ensure fair testing.
The tests were recorded over a two hour period and what they found that was that the light mode killed the battery that much that the testing in Google Maps couldn't even be concluded. Dark Mode on the other hand still had 30% of its battery remaining. Why does Dark Mode give better battery efficiency? Well that's actually quite simple. When an OLED display is showing black it means the tiny pixel on the display is completely off, not draining any reserved power contained in the battery. To the contrary, when an OLED display is at full brightness more power is being drained due to the pixels being powered on, you can probably see where this is going.
LCD displays on the other hand offer no battery efficiency since they don't use the same pixel technology as OLED display do. All Dark Mode does for LCD displays is offer a nicer sleek look.