Solar-electric hot water in a two-family home

The photovoltaic system is not located on the house, as is usually the case, but at the rear of the property. The reason for this is that in Germany, ground-mounted PV systems are permitted within a 200-metre strip along railway lines and motorways - the directly adjoining railway line therefore makes this installation possible on the owner's own property. The owner dug the required 75 m cable trench to the far end of the property himself. Fifty modules are installed on two ground-mounted racks, with another five modules on the garage roof. This gives a total system capacity of 24.75 kWp, facing south at a 15° tilt. Three SMA inverters feed the PV electricity into the home grid. This first covers household electricity consumption, then charges the battery storage system (BYD HVM 22 kWh), and any remaining electricity would - without self-consumption optimisation - be fed into the public grid. With a 24.75 kWp PV system, that would mean a lot of energy being fed into the grid for relatively little compensation. This is where surplus utilisation through photovoltaic heat comes into play: products from my-PV, the Austrian specialist in heat from photovoltaic electricity, direct this power into another sector in the home - namely heat - rather than considering only electrical power consumption.

The path to my-PV: Cloud-free integration into his own smart home

Lars K. became aware of my-PV through numerous YouTube videos. The decisive selection criterion was the ability to integrate the device into his self-built smart home without a cloud connection. The open Modbus interface on my-PV devices is well known in relevant forums and enables direct integration via ioBroker.

"I always specifically look for devices that I can integrate into my DIY smart home without a cloud. The Modbus interface on my-PV devices is well known in forums. That made the choice easy if you enjoy tinkering but do not necessarily want to solder the electrics yourself as well," says the owner.

Self-installation of the heating element

The owner was unable to find a heating contractor in his region who would install the heating element and provide warranty coverage. He therefore carried out the hydraulic installation in the 300-litre storage tank himself together with his father. For insurance reasons, the electrical connection was carried out by Solartechnik Hamm. Connecting it to the main inverter was no problem: after the appropriate menu item was selected, the AC ELWA 2 detected it without any issues.

Domestic hot water for six people

Four adults and two children live in the two-family home across two households. Today, the domestic hot water is heated primarily using solar electricity via the AC ELWA 2. The target temperature in the 300-litre hot water tank is 65 °C. The propane heating system used previously only reached 55 °C - the higher temperature level is another side effect of the changeover. "On the one hand, this allows more solar electricity to be stored as heat in the tank during the day, while at the same time increasing comfort: raising the temperature also ensures that the desired water temperature is available for the morning shower," notes the owner, Lars K.

Ensuring hot water without drawing electricity from the grid

Conventional hot water backup via the public grid is not used. If the day's yield from the photovoltaic system, or the surplus from it, is insufficient, the owner's ioBroker triggers a boost on the AC ELWA 2 via Modbus at night - but only if the 22 kWh battery storage system still has sufficient charge and the temperature in the hot water tank has fallen below 35 °C. This means warm shower water is available for all six residents in the morning without drawing electricity from the public grid.

€300 saved by switching off the gas heating for six months of the year!

With the AC ELWA 2, the propane heating system can be switched off completely from April until the end of September. Previously, domestic hot water was heated by the propane system all year round, which increased consumption accordingly. With the solar-electric solution, the need to refill propane is noticeably reduced. "We used around 15 m³ of propane per month for hot water," Lars K. gives as a guideline. "If we assume that we can deactivate the propane heating system from April until the end of September (i.e. 6 months) and take the August 2025 figure (approx. 15 m³) as the monthly average, we arrive at savings of around 300 euros for this year."
This does not yet include any possible future increases in gas prices, but is based solely on the value of the current purchases for 2,900 kWh (equivalent to 112 m³ of propane).

"That would mean we had already recouped the investment after two to three years - a great result," says Lars K., pleased.

During the winter half of the year, the existing heating system and two Mitsubishi air-conditioning systems with a total of five indoor units supplement the heat supply; when PV yield is sufficient, the air-conditioning systems are activated for cooling in summer and to support heating in winter.