Also available to read on dev.to. Hacker News discussion here
Was Blackle right? #
Many years ago, I was introduced to
Blackle is just
Blackle we'd pollute less carbon into the atmosphere — and every little helps — well that's the idea anyway — but is it true?
In 2007, when
Blackle was first launched, cathode ray tube (CRT) monitors were still popular — around
25% of desktop computer monitors were CRTs1 — and CRTs do use less electricity displaying black pixels than white pixels.2
The blog post that inspired
blackle used this
25% figure to suggest that
750 megawatt-hours a year would be saved if google went black.3
That's equivalent to the electricity use of about
53 households in the USA.4
The remaining CRT screens were rapidly replaced with Liquid crystal displays. LCDs use the same power regardless of the screen color — and so
Blackle became redundant…
That is, until OLED screens became a thing. OLED screens, like CRTs, use less energy when the screen is darker — and so, once again using dark mode saves electricity — and prevents CO2 emissions.
How much CO2 does dark mode save on an OLED screen? #
We'll do some quick napkin math, and get a very rough idea of what kind of energy savings we're dealing with here — we'll use an iPhone as an example.
Mobile Enerlytics5 (a company that researches mobile battery drain), estimates dark mode saves anywhere between
44.7% of battery life depending on what activity is being performed and the screen brightness settings of the phone.
For instance, reading The Verge on dark mode at
38% screen brightness saves
23.5% battery life, compared with light mode.
iPhone X uses a
20 watt charger and takes
~1 hour to charge.6 If you charge it fully once per day, that's
20 watt hours a day,
7.2 kilowatt hours a year.
I imagine people probably spend most of the time on their phone browsing the web, on social media apps, or messaging — that's entirely based on my own experience and not on any data (because I can't find any). So we'll make a conservative estimate that dark mode saves
5% of battery life.
In the USA
0.4kg of CO2 is emitted per
iPhone Xs on
dark mode for a year would save about
The same emissions as 31 cars.8
Dark mode saves a tiny amount of energy. It's not nothing — but with
200 million cars in the USA alone, it feels like nothing.
But a big part of this story is missing.
Built in emmisions #
The largest source of carbon emissions from smart phones and tablets is their production.
According to Apple, each new
iPhone 12 will produce
80kg of carbon.
14% is from using the device,
2% from transport,
<1% from "end of life processing" and
83% from its production.9
Producing less stuff has a much larger impact than using stuff less — and one of the reasons people buy new stuff is because their old stuff is broken.
One reason people buy a new phone is that the battery has died.
Companies like Apple make it costly and difficult to replace a battery — and so, people tend to buy a new phone instead.
Lithium ion batteries can only be used so many times before they stop working — Apple says iPhone batteries maintain
80% of their capacity after
500 charging cycles.10
The more energy a phone uses — the sooner it stops working.
So, supporting dark mode may save more carbon than a few cars.
Emissions intensity #
Increasing the lifetime of a product prevents a new product being created — which will emit carbon from it's manufacture, transport, packaging, and end-of-life.
How you calculate the effect a product's lifetime has on the environment is not immediately obvious (at least to me!)
But generally, the way it is done is by spreading the built in emissions across the lifetime of the product — then dividing those emissions by some measure that's relevant to the use of that product.
This is called
For the sake of simplicity we'll use the metric,
CO2 per year for the calculation of our smartphone emission intensities, and we'll compare it to some other things to give it context.
Calcualting how much dark mode saves #
I don't know how much dark mode increases the life-time of a phone battery. I've tried my best to find out, but there's too much uncertainty.
So instead of giving you my best guess. I've made a tool that allows you to input your best guess on the missing data.
I've prefilled it with information from the
iPhone 12 environmental report, using
100 million units, and my best guesses. The code for the calculator is at the bottom of the article.
Hopefully it'll give you an idea of the scope of this issue.
There's some factors I've left out:
- The likelyhood of getting a new phone goes up over time, new models are released, your phone has aged in other ways, software updates have stopped.
- The direct energy savings — mentioned at the start of the article.
- The second hand market — phones don't just have one owner before they end up in landfill.
- Batteries get replaced — and the carbon used to manufacture a battery is less than the device as a whole.
- Maybe dark mode also reduces the chance of screen burn-in, or increases the life-time of other parts.
And there's likely some I haven't thought of at all.
Why your website (or app) should support dark mode? #
It's our responsibility as designers and developers to minimize the damage our products cause. Dark mode does have environmental benefits — and that should be reason enough. But it's also what the people want!
- It's easier on the eyes.
- It saves battery.
- Its light is more flattering — therefore you're more likely to find love.
What's more, some browsers have the option to use
forced dark mode for websites that don't support it. This means your website may not render as you intended — and can lead to readability issues:
What have I missed? #
If you have any expertise, insider knowledge, or sources you'd like to add, let me know, and I'll happily update this article.
There's also a code pen you can play with. The code is written below for anyone who wants to scrutinize it.
var batteryLifepanBeforeValue = Number(batteryLifeSpanBefore.value)
var likelyhoodValue = Number(likelyhood.value) / 100
var builtInEmissionsValue = Number(builtInEmissions.value)
var energySavedValue = Number(energySaved.value)
var strengthValue = Number(strength.value)
var numberValue = Number(number.value)
var likelyhoodBefore = 1 - (1 - likelyhoodValue) ** batteryLifepanBeforeValue
var lifespanBefore = batteryLifepanBeforeValue * likelyhoodBefore
var carbonIntensityBefore = builtInEmissionsValue / lifespanBefore
var batteryLifespanAfter = batteryLifepanBeforeValue + (batteryLifepanBeforeValue * energySavedValue / 100) * (strengthValue / 100)
var likelyhoodAfter = 1 - (1 - likelyhoodValue) ** batteryLifespanAfter
var lifespanAfter = batteryLifespanAfter * likelyhoodAfter
var carbonIntensityAfter = builtInEmissionsValue / lifespanAfter
var savedCarbonPerDevice = carbonIntensityBefore - carbonIntensityAfter
var totalSavedCarbon = savedCarbonPerDevice * numberValue
var equivalentInCars = totalSavedCarbon / 4600
co2kg.value = Math.round(totalSavedCarbon)
cars.value = Math.round(equivalentInCars)
DisplaySearch Indicates Q1'06 LCD Desktop Monitor Market Shows Seasonal Downturn; Market Moves Rapidly to Wide Displays ↩︎
Energy Use and Power Levels in New Monitors and Personal Computers — 2002 ↩︎
Black Google Would Save 750 Megawatt-hours a Year — 2007 ↩︎
How much electricity does an American home use? ↩︎
What is the impact of Dark Mode on battery drain? ↩︎
iPhone X - Technical Specifications ↩︎
EIA — How much carbon dioxide is produced per kilowatthour of U.S. electricity generation? ↩︎
EPA — Greenhouse Gas Emissions from a Typical Passenger Vehicle ↩︎
Product Environmental Report — iPhone 12 ↩︎
iPhone Battery and Performance ↩︎
- 9 Jan 2023
- 9 Jan 2023