Unfortunately, there's usually no guarantee of either wifi or power outlets working. Most bus companies make clear in the fine print that they'll do their best to provide these amenities but that they're not guaranteed and may not be available if they have to switch buses, etc. (Airlines handle this similarly.)
Most bus wifi runs over a mobile network. It's basically the same as tethering via your phone, though the bus has a better antenna that you're sharing it with whoever else is connecting. Speed and stability varies a lot depending on the hardware in the bus, the network, region you're driving through, and amount of people on the bus trying to share the connection. If you do have a setup to tether via your own phone / internet stick / mobile hotspot, I've often found it to be faster and more reliable.
I work for Busbud, a bus search site. One thing we're doing to is to allow users to review different bus companies - many people make comments on the availability of wifi and outlets. For Paris - Turin those reviews haven't hit scale yet, but we do have general reviews for three of the companies on that route (Baltour, Eurolines, Ouibus) you could look through.