Optimal - for maximum self-consumption. a 0 - 3 kW linearly controlled water heating device for grid-connected photovoltaic systems. It uses excess PV energy for hot water heating and thus optimizes the self-consumption of the system. The linear control of the power sets the PV feed-in power to zero. It has our proven hot water boost-backup function: if the sun does not seem to be enough, the hot water can optionally be heated to the desired temperature by using energy from the public power grid. It can be installed in hot water boilers and buffer storages.