A raspberry Pi costs up to 1€/month in electricity to keep running as a server, while any used computer will cost quite a bit more. When suggesting old hardware for server use, make sure to consider the total cost, not just the cost of the hardware itself.
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.
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.