The Solar iBoost+ has a programmable timer that can be set for two times each day on a 5/2 day basis, which can replace existing timers. At the preset times, the first and second immersion will receive full grid power independent of the PV generation.
It is possible that the water is already partially or fully heated from the day's excess PV power, which means the grid operation may only need to "top up" the pre-heated water.