Does dark mode really save battery? Not on every screen

On this site, the dark theme came before the light one. Not out of ecological conviction: I code at night, and large white surfaces hurt my eyes. When I published the switch, someone replied that it was good for the battery. I wanted to know whether that was true, or whether I had just picked up a free argument.

It depends entirely on the panel

On an OLED screen, every pixel produces its own light. A black pixel is a switched-off pixel, which uses almost nothing. Showing a page on a black background with light text therefore lights up a fraction of the subpixels, and the saving is real and measurable, especially at high brightness.

On an LCD screen, the story ends right there. The backlight lights the whole panel all the time, and the liquid crystals only filter that light. A black pixel is a pixel that blocks light already produced. Dark mode then changes almost nothing about consumption, and can even nudge it up slightly depending on how the screen handles its levels.

Second nuance: brightness overrides everything else. A light theme at low brightness uses less power than a dark theme at full brightness, even on OLED. The slider at the top of the screen matters more than the palette the designer picked, which stings after an evening spent on that palette.

What dark mode really costs is readability

The real issue with dark mode isn’t energy, it’s optical. Light text on a dark background causes a halo around the letters, especially for people who are short-sighted or astigmatic, and it makes thin weights tiring to read over long pages. Maximum contrast isn’t maximum comfort: pure white on pure black tires the eyes faster than a very light grey on a very dark grey.

In practice, my dark pages use neither absolute black nor absolute white, which trims the theoretical OLED saving along the way. Images need to be slightly desaturated, shadows replaced with borders, text weights bumped up a notch. That’s twice the work of a single theme, for a result half the visitors will never notice.

So why keep it

Because a night-time reader’s comfort is reason enough, and it doesn’t need to be dressed up as a gesture for the planet. Nobody has ever saved a kilowatt-hour by repainting a blog black, and the web’s energy use is decided elsewhere: in servers, in videos, in ad scripts that weigh more than all the content on the page.

The way the argument circulates is worth a detour. A manufacturer ships an OLED panel, dark mode becomes a feature to highlight, and five years later someone tells me in a comment that my theme saves the battery of a phone whose screen I don’t even know. The technology changed, the certainty stayed the same.

When I want a phone to last the day, I lower the brightness and close the app that refreshes its ad every thirty seconds. As for the theme, keep the one your eyes ask for.