Can Dark Mode Save Battery Life and Human Civilzation?

A zillion years ago I was shown blackle.com, it's google, but with a black background.

Blackle was a black page with a google search bar. It swapped the bright white background of google with total black. Supposedly this saved a lot of electricity. Dark pixels used less energy than bright ones.

So, we all changed our homepage to blackle and saved the planet. Actually, the fad of using blackle ended as fast as it started. We found out that blackle had no impact on our screens' energy use and forgot all about it.

Low Impact Manifesto — article 9 #

The other day someone sent me a link to the Organic Basics - Low Impact Website. It's an e-commerce site designed to minimize environmental harm.

The code is open source, and comes with a manifesto describing 10 things a low impact site should do.

Number nine says a low impact site "Limits the amount of light emitted by the screen." I remembered blackle and how it had turned to be a disappointment. But the people who built this site did their research, I should do mine. So I got to googling (blackling?)

CRT vs LCD vs OLED #

It was around 2007-08 when i first saw blackle.com and most screens were liquid crystal displays.

LCD #

LCD screens are illuminated with a backlight. The liquid crystals change color, but can't emit light themselves. The backlight is on no matter what the screen is displaying.

No matter how bright a pixel is the energy consumption doesn't change. So a site like blackle has no impact on power consumption.

CRT #

When blackle first launched in 2002 the majority of screens were Cathode-ray Tubes.1

CRTs do use less electricity when the screen is black.2 CRTs also use a lot more electricity than LCDs. So blackle really did make sense.

OLED #

Today LCD is still the most popular but will soon be overtaken by OLED. OLED screens are made up of pixels that emit light independently of each other. What's more, the power consumption directly correlates with the brightness of each pixel. So the less bright a pixel, the less electricity is used.

Using a site like blackle, or setting your phone to dark mode really does save power. But how much?

How much energy does dark mode save? #

Mobile Enerlytics is a company that measures the energy use of phones. They did some experiments and concluded that dark mode saves a lot of battery life. Between 5.6% to 44.7% on a device at full brightness and 1.8% to 23.5% at 38% brightness.3

The big differences in percentage of energy-use depends on the activity of the phone. A cpu heavy task, like watching a video, uses a lot of energy, so comparatively less energy is used by the display. For simpler tasks, like reading an article, dark mode results in much longer battery life.

So, dark mode does significantly improve battery life. But how much energy is being saved? And how does that translate to CO2?

Back of envelope calculation #

I've no idea how to find an accurate answer to this. But rough calculation should put things in perspective. We'll use the iPhone X specs4 as a guide.

The iPhone X uses a 20 watt charger and takes ~1 hour to charge. If you charge it fully once per day, that's 20 watt hours a day, 7.2 kilowatt hours a year.

In the USA 0.4kg of CO2 is emitted per KWh.5 Let's assume we're saving only 5% using dark mode. So 1 million iPhone Xs on dark mode for a year would save about 144,000kg of CO2. The same emissions as 31 cars.6

1 Million iPhone X's on dark mode saves ≈ 31 Cars emissions

There are more than 200 million cars in the USA. So, I'll let you decide if dark mode is worth thinking about as solution to the climate crisis.

Personally, I think small gains like this add up. The climate crisis is a complex issue. Every source of C02 can be broken down to smaller and smaller parts. Eventually issues like this need to be solved. Luckily for us, Dark mode will happen on its own.

Dark mode isn't about saving the planet. It's about usability and to a lesser extent battery life.

As dark mode gets more popular, consumers will expect it from apps and websites. The environmental gains will come for free.

  1. Computer World - lcds gain ground against crt monitors ↩︎

  2. Scientific America – Black Is Better than White for Energy-Efficient Screens ↩︎

  3. Mobile Enerlytics – Dark Mode ↩︎

  4. iPhone X Specifications ↩︎

  5. U.S. Energy Information Administration — How much carbon dioxide is produced per kilowatthour of U.S. electricity generation? ↩︎

  6. EPA – Greenhouse Gas Emissions from a Typical Passenger Vehicle ↩︎

published
5 Jan 2021
modified
5 Jan 2021
author
Nathaniel
tags
posts climate crisis dark mode mobile