Thanks guys - really helpful.
This page (about half way down)
suggests that the board has an idle wattage of just 1.9w, which does seem oddly low for an x86 system.
So ultimately, the advice is to try and stick with ARM if I really want a capable server with low power consumption?
If I really want 8GB, does that only leave the RPi4?
The Odroid HC4 would have been perfect if only it had more than its 4GB RAM.