Can Dark Mode Save Battery Life and Human Civilzation?
Tags: climate, webdev, css, mobile
Published: 2021-01-05 18:41:28

**TL;DR — Yes, and a little bit.**

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](https://lowimpact.organicbasics.com/eur). It's **an e-commerce site designed to minimize environmental harm.**

The code is open source, and comes with a [manifesto](https://github.com/Organic-Basics/ob-low-impact-website#low-impact-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`](https://www.computerworld.com/article/2571127/lcds-gain-ground-against-crt-monitors--study-finds.html). 

`CRTs` do [use less electricity when the screen is black](https://www.scientificamerican.com/article/fact-or-fiction-black-is/). `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](http://mobileenerlytics.com) 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.

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 specs](https://support.apple.com/kb/SP770) 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`. Let's assume we're saving only `5%` using `dark mode`. So `1 million`   `iPhone X`s on `dark mode` for a year would save about `144,000kg`. **The same as 31 cars.**

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

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.**