I've been hobby fixing up several computers and even smartphones. Lots of people just throw perectly good computers away and there are lots of people who need computers right now. Here are a few thoughts.
For myself I use a 128GB Surface Go, which is pretty low wattage. Right now I'm using between 0.9 and 1.9W to write this. Of course that doesn't include charging and battery losses. Oh, I just hit 2.1W momentarily! I also have a dock which is hooked up to two more monitors, two printers (one with a scanner) and a Blu-ray/DVD/CD player/burner as well as several USB splitters. I'm not sure what my whole setup uses though.
The lowest power consumption devices are ARM based right now, although I'm sure there are some that use more than my Surface Go. There are the new M1 Macs that are ARM based. Any Android or iOS (iPadOS) devices are ARM based. There are also Windows-on-ARM laptops that are pretty low wattage.
But there's also reasons not to go the portable route. A lot of the desktop computers I've gotten are over 10 years old, or close to that. But for the most part I've picked them up for free. I usually get ones that have, or can be, upgradeable to a CORE i7. I throw some 16GB of RAM in them and an SSD. On a few of them I have needed to buy Windows 10, but Windows 7 and up are upgradeable to 10 for free still. Once all that's been done these old computers blow the doors off of my fairly new Surface GO in terms of performance.
The point is that portable computers, like laptops, tablets and smartphones, while using less power, are also more disposable, so end up costing more in the long run. Most things are soldered nowadays on these devices, so no upgrades or repairs. And since they are portable they tend to wear out faster. Don't expect a smartphone to last more than 5 years. But I'd be willing to bet that if you get a cheap desktop and it might last 20 with the way things are going.
The problem is it's kind of hard finding a combination between the two, a low power setup and a repairable/upgradeable desktop form factor. I did find
this high efficiency (35W TDP) CPU and motherboard combo but I don't think the CPU is upgradeable on it. That might shorten it's usuable life.
Also, actual hard drives, while slower, can actually outlast SSD's, especially in a desktop. If you get a device with an SSD get a good one with DRAM on the SSD. Crucial's MX series SSD's are a good choice for laptops and desktops. Seems like the internal SSD failing was the main problem I've had with older handheld devices. Five years and they're shot. Buy I've got a few hard drives that are decades old and still work.
So it seems like longevity vs efficiency.
On a similar note, I have thought about taking all my electronics, both portable and not, and figuring out how to power them all off of solar only.