The new standard in water heating

Reference project Viva House

Photovoltaic water heating is clearly the future in apartment buildings.



Client/owner/planner: DI Arno Hernach, Managing director VIVA Haus

Location: Tillmitsch, Styria
Type of building:

New construction, multi-family house, heating requirement on site 31 kWh/m²a

Year of construction and number of square meters:

2020, 4 residential units, 70 m² usable living space per flat

PV-power and orientation:

16.8 kWp on the roof oriented east/west as well as in the west-facing façade and on the balcony railing

Heat storage size:

150 l suspended storage tank with 3 kW heating cartridge per flat (total 4 flats)

my-PV product in use:

4 x AC•THOR, M1 Hot water

Space heating with infrared heating (unregulated)

System control by/with:

SolarEdge, control of the 4 AC-THORs in my-PV Multi-Mode.


Customer declaration

How did you come to my-PV?

The southern Styrian property development company VIVA-Haus describes itself as a small family business, with a total of 45 employees. Projects are managed from the greenfield to the handover of the keys. However, it is not only about cost efficiency but also about reliability. In order to also improve in terms of sustainability, new technologies are also constantly used in the projects – a practice that is otherwise unfortunately uncommon in the construction industry. Property developers are too afraid of possible failure costs and the financial risk.

Arno Hernach from VIVA-Haus takes a different view. In 2020, he built a house that did not use a conventional water-based heating system. The guiding principle was "cables instead of pipes". The final decision to do so was made after visiting an earlier my-PV reference with a comparable concept.


Obstacles/specialities during the installation

In the run-up, there were no problems with official approvals; in terms of approval, the use of solar-electric heat generation was even conducive. With regard to the energy communities, the grid operators shape the business models. In any case, it would have to become even more attractive to make full use of the roof surfaces on multi-party buildings that are available anyway, without the majority of the yield being wiped out by costs for complicated billing.

Arno Hernach would like the legislator to take the issue of photovoltaic heat more seriously. In his opinion, it is clearly the future in multi-party housing.


Brief explanation of the system – what should be mentioned?

To completely avoid circulation losses, the hot water is prepared decentrally in the individual flats. The energy for this comes from the grid-connected photovoltaic system for most of the year. The surplus information for the my-PV control is retrieved from the SolarEdge inverter. Some of the PV modules have also been integrated into the building façade, which means that appreciable solar yields can be achieved even in the cold season.

Since communal electricity consumption, for example for lighting in the stairwell, is naturally very low in buildings of this kind, a large amount of surplus energy is available for heat generation. This does not turn the landlord into an electricity trader; the complicated tenant electricity model is not necessary! Although the heat is generated electrically and the energy supply is also recorded with a corresponding sub-meter, in billing terms it is heat.

For each flat, an AC•THOR from my-PV ensures that the output of the 150-litre electric boiler is continuously modulated and thus only the energy that is currently available from the photovoltaic system is used. This makes electric heat suitable for photovoltaics, making it PV-ready.


Persons in the household – hot water demand etc.?

One of the tenants lives together with his partner and a teenager in one of the four residential units. From his point of view, the 150 litre hot water supply is completely sufficient. While the boiler is heated to 60 °C in good weather, the storage temperature is kept at least at 45 °C in bad weather. This limits the use of electricity from the public grid to a maximum and there is always residual storage capacity available for surpluses. But even during this time, he feels no loss of comfort. Should a tenant nevertheless wish to increase the minimum temperature, Mr. Hernach could even make this adjustment from his office at any time. A personal visit would not be necessary at all.


Is hot water backup with mains power used?

In this case, no hot water backup via mains electricity is activated, as only electricity from the sun is sufficient.



Personal customer opinion and resume:

Before the flats were allocated, some interested parties had concerns regarding electric infrared heating. Some of them turned down the offer because of this. It should be mentioned, however, that none of them had already had personal experience with it. The current tenants, on the other hand, are highly satisfied with the indoor climate.

The installation and set-up of the hot water system from my-PV was also simple and straightforward and took less than a day.

For Arno Hernach, decentralised photovoltaic water heating will even be the future standard in his housing projects! While he has also relied on classic piped solar thermal in the past, he now sees photovoltaics as the future. PV is easier to present economically for property developers and is also maintenance-free. In addition, in his opinion, multi-apartment buildings are even more suitable for PV than single-family houses. Even in the transitional period, very high coverage rates are possible, and for him, hot water is one of the most sensible consumers of all.




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About us


We at my-PV get more out of photovoltaics than just electricity, we move your building technology into the future. Use your solar power yourself instead of feeding it into the public grid: Electricity, hot water, heating and charging options for your e-car. With our products, you can use your self-generated photovoltaic electricity for your own needs without any detours. This way, you save costs in the long term and act environmentally friendly!



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