Only web in black or very, very dark grey, pt I: screen modes and themes

Quick thoughts on the design of this website. What are the energy footprints of the components that make and deliver the webpage. It's by no means an exhaustive analysis, but food for thought and perhpas timely as AI (guilty pleasure left) goes mainstream.

Excuse the title, paraphrased from Batman's line in Lego Movie, but it highlights one aspect to start; the screens through which we see the world (wide web).

Even a shallow dive reveals many works on energy savings from going dark on displays [1-3] and even colour selection [4,5], as well as an ever expanding range of display types, from the familiar LCD, to OLED, QLED to more recent additions of mini-LED and direct view micro-LED [6,7].

Of course, display tech started out in dark mode [8, 9] and is gaining retro pupularity. not only for the climate conscious but also for its asthetic and (possible?) ease on the eyes [3,10,11]. Key takeaways...

Dark mode only makes energy/CO2 savings on non-backlight displays, e.g OLEDs.
Typical mobile use in light mode consumes around 7 kWh a year.
Going dark on an OLED mobile saves a tiny 0.14 kgCO2 emissions a year.
But scaled to billions of OLEDs in circulation, savings reach 350 000 tCO2/yr.
That's nearly 5000 full flights from London to New York.

A dark mode delemma

Overall brightness dominates energy savings, with a near-linear decrease in battery discharge on sliding left on the brightness bar (100-0%).

Check it out on a mobile with a battery monitor from the app store (easy to to track mA on Android [12], not so on iphone). Dark mode/theme savings depend on display type and method of lighting.

Take the liquid crystal display (LCD), the slimline screen once seen as a sci-fi leap from old chunky CRTs, is now the standard commanding over a third of display shipments [13], the remainder split amongst a growing market share in (inorganic) light emitting diode (LED), quantum dot (QD) LED and organic light emitting polymer (OLED) based screen tech.

The LCD display is typically backlight by a broadband light source, the original (cold cathode) fluorescent lamp now replaced by LEDs that are combined to emit polarised "white" light.

Liquid crystals and red, green and blue (RGB) filters in each "pixel" of the screen control intensity (by alignment of LCs with respect to the polarisation) and color of light emitted from the screen.

The QLED display replaces the backlight wth blue LEDs and the filters with quantum dots (QDs), semiconductor nanoparticles that offer intense emission and size tunable colours. To the point, with an "always on" backlight dominating power consumption (over LC control), switching to dark mode has no energy saving over light mode.

By contrast in true OLED displays, each pixel comprises subpixels of light emitting polymers, each chemically tuned to radiate R, G and B light, the brightness of each controlled independently by separate contacts.

So, unlike LCD, for dark mode or themes with large areas of black (or very very dark grey) subpixels on the OLED screen are "always off" to provide maximum contrast and some energy savings.

But not much and somwhat dependent on your brightness setting. Recent studies find only 3-9% savings in battery life at 30-50% brightness and up to 40-50% at 100% [14].

Again the mobile battery app can be used to visualise the reduction in discharge rate on switching from light to dark mode, dropping on the OLED in this case (400 mA - 250 mA)/400 mA ~ 38% at maximum brightness).

More recent manifestations of LCD and QLED displays use a backlight array of mini-LEDs, each powered separately to allow localised dimming and again nominal energy savings. The micro-LED eliminates the backlight and LCD with subpixels of R, G and B micro-LEDs "viewed directly" and individually controlled in brightness should provide similar energy savings [no clear ref].

Despite the ease of scaling micro-LEDs to 8K displays, increases in overall energy consumption with higher pixel densities face regulatory hurdles [15].

A little light in the dark

And the CO2 emissions saving? Taking the mobile as an example, emissions can estimated from the power consumption under typical use and CO2/kWh conversion factors [16], the global mean measuring 0.4 kgCO2/kWh and typical use.

Assuming an average battery (old and new) rating of 3500 mAh and a typical 4 V operating voltage, a drain time of 18 h under typical mobile use in "light mode" gives consumption around 14(Wh)/18(h) ~ 0.8 Wh per h [17, 18]. That's 0.8 x 24 x 365 ~ 7 kWh/yr that converts to 0.4 x 7 ~ 2.8 kgCO2/yr (per phone).

For typical (OLED) mobile use at 40% brightness and 5% saving in dark mode [14], going dark saves 0.14 kgCO2/yr. As much saved in a year as CO2 churned out driving a (non-EV) car just under one km [19].

Pitiful perhaps, but scale to over 2.5B OLED mobiles shipped since 2019 [20] and emissions could be reduced by around 2.5B x 0.14(kgCO2/yr) / 1000(kg/t) ~ 0.35 MtCO2/yr [cf. 2, 21], assuming no users already operate mobiles in dark mode.

The footprint of flights work well for context here and London to New York works out at a neat 300 kgCO2/passenger per leg [22]. Our scaled emissions savings over the year are now equivalent to 0.35(MtCO2) / 0.3(tCO2) / 200 ~ 4800 flights, with a typical capacity of 200 passengers.

No claims on accuracy here and clearly assumptions propagate uncertainties in these numbers, but food for thought and the potential for reducing the "usage" footprint could increase as the OLED (and microLED) share of the smartphone market increases going forward. Of course, no account of the emissions intensity from carbon embodied in production, distribution, and disposal/recycling is included.

What's clear is materials and manufacture make up significant emissions costs, but these vary widely, from a "sustainable" phone (~30 kgCO2) [23] to higher end (~80 kgCO2) [24], and the "usage" component of the phone's footprint often accounts only for battery use and not the energy/CO2 cost of calls n clicks on the network [25, 26].

Cost of the click continues in pt II...

*Links accessed at 05/02/2024

Dr Mark Osborne

Only web in black or very, very dark grey, pt I: screen modes and themes

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dr.mark.osb@gmail.com

Brighton, UK