That's the frequency, Kenneth

Wireless light switches are marvellous things. This one controls the side lights in a room; elsewhere a similar switch provides bathroom lighting that can be reached by the smallest ankle-biter without recourse to waking the parents. Most of them work over fairly short ranges, as there's no significant antenna and they're generally forced into the 433 or 868 bands (433MHz in this case), but for the very lazy it is an amazing thing.

This one does have an antenna, and a decent range considering it is operating at 868MHz where transmission power is heavily regulated. It came from the local electricity company and clips to the mains wire entering the house. It transmits the power consumption to a screen which can be put on the shelf in the kitchen to guilt residents into burning less energy.

It also works as a reminder when one has left the electric hob on, or when one's Bionaire heater has lost contact with its remote thermometer and is pouring heat into the shed in the hope of finding its friend again. ®