This is how things are done in NZ already. Most electricity company have contracts with two tarrifs (sometimes even more), one normal one and a second significa

This is how things are done in NZ already. Most electricity company have contracts with two tarrifs (sometimes even more), one normal one and a second significantly cheaper one where the circuit is controlled by the power company and they can turn off things when there is hight demand. Typically you'd put your water heater, electric car and other things on this circuit. I the more long term, exchanging resistive water heaters with heat pumps is the way to go (and already done throughout Scandinavia)