Myth: Does dark mode save battery life?
There is a prominent myth in the tech community that by simply flipping the switch known as "Dark Mode", you can save battery life because your computer or device's screen is not drawing as much power.
The Bad News
This is a misconception on how display technologies work. In general, the common laptop or phone is powered by a LED Backlighted LCD. The LED backlight used is one large dimmable white light panel that sits flush behind the screen. The backlight stays the same user-set brightness no matter what content is displayed on the screen. With an LCD panel, the color black and the colors adjacent are generated by placing a filter over the backlight.
The Good News
Computers and phones are evolving to a new technology called OLED. With these displays, each pixel is individually powered. Pixels that display any color other than black are turned on, while pixels that are black or near-black are turned off. This method of the display managing power consumption can be vastly more power efficient than LCD.
The Ugly News
OLED Displays still cost more to produce and manufacture than any LCD display currently in production. Most modern flagship smartphones are using OLED or a variant of the technology. This pricing also scales with display size; many consumer televisions sold in superstores today are still using LED-backlighting with LCD panels. Fortunately, due to their massive display size modern televisions require an array of LED backlights, often in rows across the display. TV manufacturers have used clever post-processing techniques to analyze the image shown and control the individual LEDs. This is a clever way to not only save energy, but offer a compelling alternative in image quality to costly OLED TVs.
Also, while OLEDs can save power based on the content displayed, it often depends on the use case and what you are displaying on a daily basis. So generally to reap the most reward, you would want to have a minimal, icon and widget barren, black background homescreen. You'll want to set your system theme to dark mode, and apply dark mode in frequently used apps that support it. Some may not have full support for your OS or not have support at all; if you are dedicated to battery life and nothing else it might be time to find an alternative that does support dark mode.
The Conclusion
If you have a smartphone made within the last 3 years, chances are you are equipped with an OLED display and can theoretically save a bit of battery. In most cases for laptops and tablets, your display is an LCD and there's not much you can do with or without dark mode. In both cases, substantial battery improvement can be more readily found in changing device usage habits, like turning brightness down or turning off wireless communications like Bluetooth and WiFi when not in use.