The electric boiler provides hot water for showers (critical use) as well as for other uses (washing hands, dishes, etc.). It is an excellent “thermal battery” with an immediate response and a lot of displacement potential.
The electric boiler is permanently plugged in and heats up shortly after cold water is obtained, irrespective of the price of electricity at that time. After the morning showers, the user may not need hot water until after lunch or dinner, and does not need it at 60°C.
The user defines the critical usage time. The rest of the time the system runs on PV surplus or heating during the cheapest hours.