Originally Posted by TheNinjaPirate:
“...Only problem I could forsee is that if your upstaris and downstairs mains is on a different ring main you *may* not get networking across them - I would look into that.”
Most UK domestic homes are run on a single-phase system. Within "your phase", you stand reasonable chances of communication, if none of the many possible impairments get in your way: switch-mode power supplies with the "wrong dirt", any slip ring or inverse-speed motor (i.e. your blender, vacuum cleaner, lawn mower, ...), energy-saving light bulbs, etc. You may also be affected by someone else on the same phase - the first likely stop to your communication is the next transformer.
Up to the next upstream transformer, you'll be sharing your data with the world. Your electrical meter is unlikely to attenuate the signal enough to keep it in your home.
So, in terms of reliability or security, you are unlikely to gain anything over WiFi.
My advise is to use regular network wiring where possible for cost, reliability, performance and security reasons.