Computer efficiency numbers – useful rating or greenwashing?

In a recent newsletter, well known scientist Katherine Hayhoe mentioned a listing of the top most efficient computers – while efficiency is something to consider in your purchases, is there really a big difference in which laptop you buy and use?

For this list she says “The cheapest energy is the energy you don’t use; that’s why I also use the most energy efficient laptop, according to this list.” Now wattage is indeed W=V*A for voltage and amperage, but that listed rating is the rating of the laptop and not what it is actually using. Plug the computer into a Kill-a-watt or similar power monitor and you will most certainly see a varying amount less than the rated power being pulled from the outlet – especially if it is full or nearly full battery where it will not be charging faster.

Second, the wattage will depend on the adapter, and with the ubiquitous USB-C nearly any computer can charge off an adapter that is even low as 18w “cell phone charger” (overnight, when turned off for hours). So a 30w rated adapter that comes with it doesn’t necessarily mean it is an efficient computer. Let’s look at some actual numbers from various computers and see what a more efficient computer actually may mean over a work week:

VoltsAmpsRated Wattage V*AActual measured watts (high value, varies)8 hr day (w*h)5 days (Kwh)
Dell XPS (10 yr old)19.54.6290.09504002
Dell XPS infinity-edge model19.56.67130.065806403.2
Apple Macbook 2022 included adapter302401.2
Macbook 201920.24.386.86806403.2

Now if the rating is correct with the Macbook 2022 and it takes 30w or less with its charger (and charges more than it discharges while in use), then it is significantly less power. But how much is the 2KWH that you will save in a work-week’s use on the more efficient laptop? (that is assuming a constant charge/usage rate which would actually not be applicable while sitting with full/nearly full battery!)

Comparing Apples and Oranges

2KWH is enough to drive an electric car maybe 6 miles more or less, depending on car and conditions (you can calculate your own values here). Deciding to combine trips, or cycling to a local place just 3 miles/5km away rather than drive would save approximately 2kwh of energy.

Kid car
No not that type of car

Suppose you don’t drive or bicycle much where you live… here’s another comparison. According to GE, their clothes driers may use up to 5600W of power – if you cancelled a half hour drying cycle and air dry/use a clothesline once a week that would have the same effect.

Do you enjoy tea/coffee? If you have a big tea party and have an electric tea kettle heating fully on for 90 minutes or so that would be 2KWH (at 1500w). If you decided to make a big jar of sun tea or other cold drink instead that would also save 2KWH.


All in all it seems there are other ways you could save energy easier than switching to a lower power computer. The other suggestions such as powering off and recycling computers are good, but ironically choosing a lower power laptop such as a Chromebook will probably make it underpowered/obsolete and replaced within a few years, negating minimal efficiency gains.

I’m surprised that nowhere in the article was GAN charging mentioned. This new type of charger available in recent years makes the charging block both smaller and more efficient. A charger like the Pinepower would make a much easier (and cheaper) upgrade than a computer replacement for anyone with limited power or expensive electricity prices.

What ways are you reducing energy expenses this summer – and do you agree with the use of minimalist Chromebook/Macbook-air computers? Send a note in the comments area:

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