It depends on your screen type
Phones use one of two display technologies. On an LCD screen, the whole panel is lit by a backlight that stays on regardless of what is shown, so a dark wallpaper saves almost no power. On an OLED or AMOLED screen, each pixel makes its own light — and a black pixel is simply switched off. That is where the savings come from.
Why black pixels matter on OLED
Because an OLED black pixel uses essentially no power, the more truly black area on screen, the less energy the display draws. A deep-black or "AMOLED" wallpaper with large dark regions can measurably reduce screen power use, which is the single biggest battery drain on most phones.
How much does it actually save?
The effect is real but modest for a wallpaper alone, because you mostly see your wallpaper on the home and lock screens, not while using apps. The bigger wins come from system-wide dark mode plus a dark wallpaper, and from an always-on display that shows mostly black.
When a dark wallpaper helps most
You will notice the most benefit on an OLED phone, with dark mode enabled, and an always-on display. In that setup a black wallpaper keeps huge portions of the screen switched off throughout the day.
Beyond battery
Even when the savings are small, dark wallpapers have other perks: they are easier on the eyes at night, they make colorful app icons pop, and they hide smudges and fingerprints. Browse PicLoy’s dark collection if you want the look as well as the efficiency.