Dnk,
You are correct.
Power companies here limit the operation of storage Hot-water cylinders by what is known as a Ripple Control system.
Effectively all it means is, the PoCo induces a series of tones (anywhere from 500 to 1500Hz) into the supply lines to open and close a ripple relay (actually a type of impulse relay) at each customers installation, thus controlling the amount of time the load is energised.
It is generally referred to as "Night-Rate" as the supply is usually switched on at 11pm and switched off again at 7am.
Night-store heating is also controlled in this manner, except this requires an extra meter in the meterbox and an extra channel module in the ripple relay itself.
