A small IoT-like peer can be easily composed to cover for an electrical phase. Two would as such be needed for a three-phase house, if we assume the router covers the third. (If the third phase is out then the remaining two cannot handshake anyway.)

• IKEA Småhagel USB charger
  • Any charger will technically do, but one with a port on the side and with a flat top is highly recommended.
• System-S 90 degree angled micro-A USB charger cable
  • Likewise any cable will do, but a short one with an angled connector can decrease the physical footprint.
• Raspberry Pi Zero 2W
  • If you buy a clone, make sure it supports wireless networking. (The non-W Raspberry Pi Zero 1 does not.)
• Raspberry Pi Zero case
  • Any case with a flat base will do.
• Velcro adhesive strip, 3 cm wide if possible (a narrow one will make it wobbly)
  • From your local hardware store. Don't buy the type requiring you to sew it in.

Small form factor.

Front. Your style of plug may, of course, vary.

Back. Given a short cable it doesn't take much room at all.

Right side.

Left side.

Bottom. The velcro can be seen holding the Pi to the charger.

European sockets are recessed, likely so as to make it impossible to touch one of the plug leads as you insert the plug into the socket. If you place the velcro in a way that places the Pi unit too close toward the plug, it may hit the socket wall and prevent the plug from being inserted fully. The solution is just to place the velcro a bit further back.