FAQ for private customers: Building technology with photovoltaics



Questions about photovoltaics and solar-powered houses in general.

Self-consumption means nothing more than using the electricity generated by the photovoltaic system yourself. The electricity from the system is made available directly to the consumers in the house. Self-use describes the energy requirement that a household needs to cover the power supply as well as the heat and hot water requirements.

Self-sufficiency means independence and energy self-sufficiency refers to the independence from external energy sources. If you do not use any electricity, gas, heat or other energy sources apart from those that you generate yourself in the house, then this is called an energy self-sufficient house. Self-sufficiency in connection with photovoltaics describes independence from the public power grid.

The degree of self-sufficiency refers to the portion of your power requirement/needs that you are able to cover with your photovoltaic system. The self-consumption rate shows what percentage of the photovoltaic electricity you actually use yourself.

The sun supplies around 1,000 kWh of energy per 5 square meters of photovoltaic modules per year. A system of 50 square meters thus produces approx. 10,000 kWh/year and, in purely mathematical terms, covers the annual electricity requirements of an average family of four by a factor of 2.5. You can see how high your own consumption actually is based on the annual statement from your electricity provider. You can simulate the corresponding calculations with the help of our Power-Coach.


A very good question! There are several approaches to increasing self-consumption. We have summarized what we think is the best way to proceed in the following article.

No, at least not right away. We advise to first insulate the facade, windows and roof and only then to deal with the topic of heating with photovoltaics.

In order to be able to heat a house with photovoltaic power, the building shell must be very well insulated with a maximum annual heating requirement of 50 kWh/m², based on the local climate.

A photovoltaic system with at least the following rated output makes sense for heating with photovoltaics:

 Heating demand factor [kWp/m² living space]

 >45-50 0.1

 >40-45 0.08

 >35-40 0.07

 >30-35 0.06

 >25-30 0.05

 <=25 0.04

Here is a concrete example: With 150 m² of living space and a heating demand of 40 kWh/m² per year, a minimum output of 12 kWp photovoltaics is required (=150 x 0.08).

The duration of the warm-up depends on several factors (more details on this in the blog post Heating element combined with photovoltaics). The theory says the following: To heat 1 liter of water by 1 degree, you need 1.16 Wh. A 300 liter water tank needs about 17.4 kWh of energy to be heated from 10 °C to 60 °C. A 2 kW heating element needs about 8.5 hours for this.

The answer to this question is simple: By ensuring that you have used the most energy that you can from your PV system, you save yourself from potentially having to re-purchase the same energy from the grid at a higher rate. The very low feed-in tariffs provide another good reason for increasing self-consumption.

If you use energy yourself, then this corresponds to the value of the energy source that is not required. With gas, for example, a kWh costs up to 0.30 euros, plus the efficiency and maintenance costs of the gas appliance, i.e. another 2 to 4 cents. You do not have to pay these prices if you use your own energy. Depending on the heating system, you also replace fossil energy consumption, improve your CO₂ balance and thus your ecological footprint.

Our products are not only much simpler, but now also significantly less expensive than solar thermal systems. This applies to both single-family homes and apartment buildings. There are many advantages over solar thermal energy:

  • By eliminating pipes, you save up to 90% valuable copper.
  • Our ELWA does not require costly elements such as pumps, valves, expansion tanks, anti-freeze mixtures, insulation, etc.
  • Photovoltaic heat generation works even with low radiation intensity.
  • Solar-electric heat generation avoids start-up processes that may involve losses.
  • Our devices are extremely low-maintenance. For example, there is no need for an annual check of the antifreeze mixture.
  • There is no material fatigue during system downtime.
  • The efficiency of our products is independent of the storage temperature. As opposed to traditional solar thermal systems where the efficiencies of water-based heat generators decrease rapidly as the water temperature rises.
  • When outside temperatures are low, photovoltaics work more efficiently.
  • The energy transfer from the roof to the water tank is almost loss-free.
  • The installation of our photovoltaic heating element in the storage tank is quick and uncomplicated and is possible even when the storage tank is full.
  • Our ELWA has practically no energy consumption of its own.
  • With AC•THOR and AC ELWA 2, excess photovoltaic energy can be fed into the power grid after heat generation is complete.
  • In residential construction using several smaller hot water storage tanks, our products can be implemented in a decentralized manner helping to avoid distribution losses over extensive pipelines.
  • With our solutions, solar energy is only converted into heat where/when it is actually needed.
  • Photovoltaic modules have been getting less and less expensive for years.


Solar thermal energy is often incorrectly said to have an efficiency of 80 percent. However, this is only a snapshot that is obtained on the collector test stand (without any heat emission), in other words: the value has no practical relevance!

It is much more objective to compare the annual energy yields of both technologies. Well-functioning solar thermal systems with flat-plate collectors provide around 350 kWh of heat per square meter per year. A photovoltaic system with the same area provides approximately 200 kWh of electricity per square meter. For a typical hot water system in a family home, you need 6 square meters of thermal collectors or 10 square meters of photovoltaic modules for the same output. At first glance, the solar collector seems to have an advantage here, but that is deceptive! This difference is only relevant if there is not enough space on the roof. If space is not an issue, photovoltaic modules have the edge, because

  • The sun shines for free – make full use of the space!
  • Photovoltaic modules generate electricity that you can use in many ways and not just heat.
  • Photovoltaic modules are significantly cheaper to buy.
  • Compared to solar thermal, it offers enormous savings in resources.
  • The maintenance requirements are significantly less demanding.

Read more here for comparison: Solar thermal or photovoltaic?

A linearly controlled heat generator is an electrical heat generator where output can be set at as needed from 0 to 100%. These heat generators are also not switched on and off by a thermostat. This linear function is essential for operation with photovoltaic power, as the available power changes constantly due to solar radiation and other consumers in the house.


The benefits are numerous: installing cables instead of pipes means much lower investment costs in new structures. The restoration causes significantly less interference with the structure of the building than the replacement of a water-based heating system. Furthermore, photovoltaic returns can be implemented in all energy sectors of the house, including electricity, heat, and electromobility, using our products.

Our my-PV products can be combined with some e-charging station solutions. You can find up-to-date information on this in the article: Compatible charging stations – what is possible with my-PV?



FAQ about our photovoltaic heating elements

Our ELWA does not require any additional components, apart from the photovoltaic system (without inverter) and a heat storage tank in which ELWA is installed.

No, because it is an AC device. AC ELWA 2 is plugged into a normal socket. From this, however, it only takes as much power as is currently available as a surplus.

Since there is no intervention in the components of the photovoltaic system, a combination with all commercially available grid-connected PV systems is possible.

No, a combination with a battery is not possible. Our ELWA was developed for thermal storage in hot water boilers or heating buffers and uses the power directly from the PV modules. Parallel operation with battery is not intended on the DC side. For use with battery storage systems, please use our AC ELWA 2, our AC•THOR or AC•THOR 9s.

Thanks to the standard 1.5 inch thread, ELWA and AC ELWA 2 can be easily screwed into the tank. This is possible even when the storage tank is full. You can see how it works here.

Yes. Depending on the number of heating element points available, several devices can be used. However, 2 ELWAs can also be connected to a module string and interact with each other according to the master/slave principle. With a larger heat accumulator, thermally optimized loading is also possible. The upper device has priority, which means that the desired target temperature is available more quickly.


With our ELWA, no electricity is fed into the grid. Any excess energy remains unused, as with a solar thermal system. The main technical difference is that, compared to solar thermal energy, this does not result in any material fatigue.

In addition, the proportion of excess energy with the ELWA is very low. In practice, it is 5 to 8 percent. That would be about 100-150 kWh per year for a 2 kWp system.

The extra work is not worth it if you require an inverter, an electricity meter with meter rental, a feed-in point, and a grid connection.

Nothing, the device continues to run normally. Only the optional hot water backup is not available.




Frequently asked questions about AC•THOR and AC•THOR 9s

You can use our AC•THOR in all houses with a photovoltaic system and hot water storage tank that uses excess PV to heat domestic water. In buildings with low heating requirements, you can also use photovoltaics to support electric room heating.


AC•THOR or AC ELWA 2 need information about the power that is fed in at the metering point to regulate the excess photovoltaic energy. This comes either from the my-PV WiFi Meter or from a compatible energy management system (inverter, smart home or battery storage). The data transfer itself takes place over the local network, so proper setup of the network is essential for it to function.

Solar-powered direct heating for room heating only makes sense in buildings with low heating requirements. By this we mean objects with a specific energy index of a maximum of 50 kWh/m²a.

Depending on the heated area, there is an annual energy requirement. In a single-family house, for example, this should be in the order of around 4,000 kWh, i.e. roughly the same amount of energy that is required in such a property for electricity and hot water generation. The power of the photovoltaic system should then be in the order of 8 to 10 kWp.

A PV system size of between 3 and 10 kWp makes sense for a detached house, depending on whether only hot water is to be prepared or whether space heating is also to be supported.


For energy-saving single-family houses with, for example, 10 kWp photovoltaic output, the electricity requirement can be covered by photovoltaics without any problems. A heat pump, on the other hand, can only generate heat. It does not contribute to the electrical loads in the building. In contrast, photovoltaics supplies normal electrical household consumers with priority over heat generation and thus makes a major contribution to reducing operating costs.


No, it is an AC unit. The AC•THOR is plugged into a normal socket. From this it only draws as much electricity as is currently available as a surplus.


The AC•THOR is sometimes misunderstood in this way. But it is not. Technically speaking, it is an AC power controller. It continuously regulates the output of electrical heat sources depending on the photovoltaic energy supply and heat demand.


Yes, the system with the AC•THOR is one of the least expensive solutions on the market in terms of initial cost as well as operating costs. In existing buildings, it relieves your existing heat generation system significantly, in new buildings it can even completely replace conventional water-based building services.


No, of course you can also use the AC•THOR for pure hot water preparation. The connection of an electrical heating system is optional. Water-based space heating can also be supported. The heating element is then installed in the combi or buffer tank or 2 heating elements (hot water tank, heating buffer) are connected.


No, both devices can continuously regulate the power on their own.

  • Advantage of AC ELWA 2: It is a fully integrated solution and an additional heating element is not required.
  • Advantage of AC•THOR: Almost every electric heat generator can be continuously regulated.
  • In addition, AC•THOR can also linearly control two heating elements one after the other. This means that stratified charging with two heating rods can be implemented at minimal cost.

Yes, with the my-PV WiFi Meter's 3-phase transducer. This recognizes how much surplus is currently available at the house connection and continuously sends the information to AC•THOR or AC ELWA 2.

Yes, conventional, water-based heating systems can also be supported by AC•THOR. So you don't have to start the pellet boiler or the heat pump for hot water preparation in summer.

Combined operation with heat pumps is also possible: Since the output of heat pumps cannot be controlled at all, or at least not arbitrarily, AC•THOR takes over the precise control of the excess output with a heating element. This optimizes your self-consumption share.

Thanks to the intelligent surplus management, AC•THOR draws less electricity from the public grid than is the case with heat pumps. For anyone who wants to build or renovate a house, AC•THOR offers considerable savings potential: the building services can be installed in the smallest of spaces and compared to heat pumps you save up to 30% of the acquisition or operating costs.




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Then take a look at our expanded FAQ database, where you will also find questions and answers on technology and troubleshooting as well as data sheets and assembly instructions.

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A-4523 Neuzeug

+43 7259 39328

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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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