Where I live, our local electricity distributor (Orion, Christchurch NZ) controls hot water heating using a short pulse-train signal transmitted over power line

Where I live, our local electricity distributor (Orion, Christchurch NZ) controls hot water heating using a short pulse-train signal transmitted over power lines. At home, a relay device colocated with our electricity meter picks up the signal, and energizes our hot water cylinder's circuit at 9pm, when our night-rate tariff kicks in. Another pulse is sent at 7am instructing the cylinder's circuit to turn off. The distributor also shuts off the hot water for one hour during the night, spreading this out over ten or so load blocks in their network. Not only does this result in load shifted to night when it is cheaper, it also means the load profile across the whole South Island is beautifully flat. This technology is comparatively ancient, and we call it simply "ripple control". My wife and I shower at night and we find we have all the hot water we could possibly need.