It isnt 2005 anymore. Old hardware nowadays is "Lot of 5 LENOVO THINKPAD YOGA 260 i3-6100U 4GB Ram 128GB SSD Cracked Screens" coming down to $40 a pop. 2x TDP (15W vs 7.5 of rpi4) but at the same time >2x CPU power, >10x GPU power. Not to mention you get real SATA SSD storage, real DDR4 sodimm slot, keyboard, and battery backup.

Not anywhere in Europe. People tend to sell used stuff for more than the new. Maybe I'm looking in all the wrong places.

I find that Microsoft Authorised Refurbishers are a good place to start to find cheap old corporate machines - eg I can typically get something like an Intel i5 8500T based Lenovo/Dell/HP mini PC with RAM and SSD for less than £200.

I actually bought a Lenovo M910Q with i5 6500T, 256GB SSD and 8GB of RAM for ~£100 in the last year.

It'll absolutely demolish a Pi 4 at CPU & GPU performance, can take much more RAM than a Pi, idles at <10W on 230V mains, you get the bonuses of easy software compatibility as it just runs any mainstream Linux distro & and it runs x86_64 code, it has real storage interfaces (SATA & NVMe), and has an Intel NIC.

For home server uses, they're really nice machines.

The keyword here is "cracked screens".

A laptop I use for this function draws 10W on average, and that is without any additional tooling like:

https://github.com/AdnanHodzic/auto-cpufreq

TDP is not something you should look at.

It's just the top, not how low it can go on idle. My VM server has ~35W TDP but gets 20W with SSD/dual NIC and acting as WiFi accesspoint. CPU usage is less than half of that, at those levels motherboard is often the element with highest power consumption and it is hard to get data on which motherboard is most power efficient.

Probably easier with laptops as in theory you can just get run time on battery, battery size and calculate power usage off that.

Right. And then go back to your gaming PC with the 250 watt GPU running inside. /s