A common question is, how to connect the Solar Station and Solar Plug for heating the boiler.
This issue is somewhat more complicated, so I will first describe the situation I dealt with. In our household, we primarily heat domestic hot water (DHW) using a heat pump. The heat pump has its limits, heating the water to a maximum of 45°C. In case of heat pump outages (during severe frosts), the storage tank is equipped with a heating spiral (direct heater) with manual control. The spiral can heat the DHW up to 65°C. Occasionally, this preheating is also used to kill Legionella (more here).
It is precisely this additional spiral that I used to optimize the heating of DHW from the solar panels. I connected a standard 230V plug to the spiral and connected it through a smart plug SOLAR PLUG 3600W. In SOLAR STATION, I set a switching rule based on the battery reaching 80%. Whenever the main household battery exceeds 80%, the plug is turned on, and the storage tank is preheated to 65°C.
If the following days are gloomy, we can use this really hot water for the next 2 days. If the hot water drops below 40°C, the heating through the heat pump turns on. In the case of a series of nice days, hot water is maintained in the storage tank, thereby saving the heat pump compressor.
There is no need to save every watt from surpluses. The battery rule works sufficiently. If the battery reaches 80%, it is usually just before the peak of the day when solar energy is at its highest. Conversely, the battery's charging power decreases. It does not matter at all that the battery slightly discharges during DHW heating. Usually, before the PV production drops, the DHW is reheated, and the thermostat turns it off. The battery manages to recharge to full again. The plug remains turned on but no longer draws any power. When the battery drops below 80%, it turns off completely - sometimes in the evening. Personally, I have been using this system this way for the second year, and it behaves predictably - we do not turn on the heat pump during the summer.
You can monitor the saved kWh on SOLAR STATION on the tile designated for SOLAR PLUG.
Yes, you might argue that the heat pump is more efficient in heating water. But if electricity is free, then even 3 x 0 = 0. I based this on a situation where my heat pump does not allow heating the water above 45°C.
From my example, I would like to draw the following conclusion. For truly optimal solar heating function, it is necessary to have a bivalent heat source in the boiler. The primary source (gas, heat pump, spiral) should heat the hot water to a comfortable temperature. Whether the sun is shining or not, you simply need hot water. The secondary source - the spiral, can then be connected through the SOLAR PLUG for preheating above the comfortable temperature.
That's nice, but I have a boiler with one spiral, what now?
Although it is possible to connect it, logically it won't be optimal. You can turn the spiral on either during the so-called night rate (HDO) or using solar surpluses. However, there is only one thermostat that turns off the preheating, and you do not have the ability to influence two different temperatures. The optimization then lies in the consumption of hot water. At night, the water is heated for the morning from the electric grid, at a cost. This cannot be avoided. However, during the day, you can at least heat the hot water for the evening using photovoltaics.
Connecting in this situation is more complicated - it is necessary to use the SOLAR PLUG 3600W PRO. I recommend having it installed by an electrician; the PRO series requires professional installation. The SOLAR PLUG 3600W allows the connection of up to two external switches (SW1, SW2). Therefore, you can switch the relay using logic through SOLAR STATION, using HDO, or with a manual switch.
But I have a three-phase spiral.
Currently, I do not offer a solution for 3-phase spirals. However, we have experience connecting the SOLAR PLUG 3600W PRO "through" a 3-phase contactor. Another option is to split the 3-phase spiral into individual phases - if connected in a star configuration and each phase is switched either from the PV system or HDO. However, I reach the limits of my knowledge here, so I present this merely as inspiration for a solution.
In conclusion, I would like to state that it is not possible to heat domestic hot water year-round using PV, but significant savings can be achieved through preheating. I would be glad if you point out any mistakes or inaccuracies. I am happy to revise the article for future readers.