The embedded electric heating element generally works via an integrated thermostat, and is independent of the solar controller.
It is not necessary to install a device for monitoring peak/off-peak hours. However this may allow you to pay less for your electricity (using the electric heater only during the night for example).
In addition, limiting use of the electric heating element to night hours will maximize hot water heating by the solar panels during the day, which makes it possible to take better advantage of the solar panels.
* NB: To ensure proper operation of the solar hot water system, make sure that the electric heating element is positioned above the solar heat exchanger in the hot water tank. This will prevent the bottom of the tank from being heated electrically, thus allowing solar heating. Care must be taken not to install the heater at the bottom of the tank: this would prevent the solar circuit from working properly. Most well-designed solar hot water tanks respect this constraint.