Intel actually went and created a line of low power CPUs to compete with ARM (though their consumption is still twice as much) such as the N100, so you can get quite a bit of CPU power in a modern CPU (so, with hardware video decoding and encoding for all modern codecs).

Also with the transition from HDD to SDD, the power consumption in that side when merelly reading data (which is what's needed to seed a few hundred torrents) is way lower.

I actually have a Mini-PC with an N100 that's a TV Media Player, torrent server with an always on VPN (also another nice thing in modern CPUs - the crypto functions implemented in hardware), NAS and home server for a few more things at that tends to run at less than 20% load and uses very little power.

I don't think the mainstream desktop CPUs, even the notebook ones, are quite as power efficient as those things, and if you add a dedicated graphics card, it's even worse.

Before the current mempocalypse it was actually cheaper in the mid and long term to just buy a brand new Mini-PC with an N100 (mine was €130) to use as an always ON homesever than use an old desktop, purelly because of the massive power savings.