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Heating element combined with PV system Q&A part 1

The time of energy transition has arrived, with the current climatic, geopolitical and economical developments. We at my-PV have been working for years to establish the solar-powered house as a vision for the future. The signs have never been as good as they are today. But what is a solar-powered house? This article aims to answer this and many other related questions.

What is an immersion heater?

There are different names for immersion heaters: heating cartridge, heating element or heating filament/rode are three of them. Immersion heaters heat liquids and are perfectly suitable for energy conversion.

From a physical point of view, an immersion heater is an ohmic load, i.e. a device that draws energy and fulfils two properties at the same time:

  • The device behaves like an electrical load: These can convert electrical energy into other forms of energy or other forms of electrical energy.

  • The device behaves like an ohmic resistor: Ohm's law states that voltage and current are proportional to one another at all times.

An immersion heater is a heating device that converts electricity directly into thermal energy at a ratio of 1:1, i.e. with 100 percent efficiency.

Immersion heaters can be quickly and easily integrated into existing boilers and buffer storage tanks. It is important to have free space in the storage tank and to choose the correct installation length for the heating element. With right flange and threads in the appropriate diameter, almost all water heater tanks can be equipped with it.

Where and when does an immersion heater make sense?

Immersion heaters are powered by electricity and heating with energy from the socket is comparatively expensive. On average, a person in Austria and Germany (the figures are very similar) uses around 30 to 50 litres of hot water per day. It takes about 800 kWh of thermal energy per year for each person. From an economic (and ecological) point of view, an immersion heater makes sense if the required electricity does not come from the grid but rather from photovoltaic system.


Immersion heaters operated with photovoltaics are a simple way of using the electricity you generate yourself from the roof instead of feeding it into the grid. This way, continuously available solar energy is converted into electricity and this is converted back into thermal energy in the water storage tank. Installation is very simple because cables are laid instead of pipes. That way you can increase your level of self-sufficiency easily and very quickly and at the same time save CO2, since you do not need a fossil-based system to generate hot water.

Depending on the location, inclination of the system and its output, the combination of photovoltaic system, immersion heater and hot water storage tank usually cannot supply the house with hot water all year round. However, the electricity from the roof is sufficient to cover a large part of the heating requirement and to send the regular heating system into summer break from April to October. All in all, hot water generation with a photovoltaic system and an immersion heater can save costs – a strong argument given the rising energy prices! You can test and calculate the costs in comparison with the my-PV Power-Coach.

Further reasons in favour for the dream team mentioned above: The immersion heaters for the heat accumulator are almost maintenance-free and inexpensive. They can usually be retrofitted quickly and easily, but can also be taken into account when planning the storage tank.

In contrast to solar thermal systems, this concept does not have a water-carrying circuit. This means that you do not need pumps, pipes or heat exchangers. This way, you keep the acquisition costs, the maintenance effort and the operating costs low.

An immersion heater powered by photovoltaic electricity, such as our ELWA, AC ELWA-E, or one connected to an AC•THOR, allows you to turn off the heating in summer. In winter, however, it cannot replace the heating. In low-energy houses, however, a complete heating system can be implemented with photovoltaic energy. All information regarding this can be found here.


How economical are photovoltaic immersion heaters?

In the previous paragraph, we have already explained profitability and savings potential of the combination of photovoltaics, immersion heater and hot water storage tank. But it goes even further: The hot water storage tank in the solar-powered house serves as a "day storage" for excess solar power.

Since feeding electricity into the grid does not pay off, it is primarily used for your own needs. First of all, of course, there is direct consumption by the electrical household consumers, followed by the generation of PV heat. The size of the existing photovoltaic system affects the possibilities and the degree of profitability of your photovoltaic building technology. With our my-PV Power-Coach you can get a first overview and calculate the degree of self-sufficiency you can achieve thanks to photovoltaics and an immersion heater.


What types of immersion heaters are there?

The simplest version of an immersion heater only has an on/off switch. Other variants can gradually regulate the power of the immersion heater. And the most convenient solutions, or immersion heaters that are best used with photovoltaics, have linear control categorized as power diverters (as with the my-PV devices). The power of the immersion heaters in the water tank is adjusted to the available surplus photovoltaic in a manner that is precisely to the second. Only then does electric heat become photovoltaic-capable, only then does it become "PV-ready".

As already mentioned, an immersion heater for a heat storage tank is largely maintenance-free, which of course reduces the operating costs. The electricity required is obtained from the photovoltaic system. The power consumed by the immersion heater depends on the available PV surplus and the amount of heat drawn from the storage tank, as well as the storage volume and its thermal losses.

Good to know: Our AC ELWA-E is an immersion heater linearly controllable from 0 to 3 kW for grid-connected photovoltaic systems. ELWA is a 2 kW immersion heater and perfectly suited if you want to use your solar power exclusively for the producing hot water (keyword isolated system for fans of self-sufficiency). In combination with the AC•THOR or the AC•THOR 9s, even immersion heaters with an output of 9 kW can be made controllable.

However, the units offered on the market differ not only in their output, but also in the way they use the electricity. So-called DC immersion heaters work primarily with DC voltage and process the solar electricity coming from the roof directly. However, they can also be operated with electricity from the power grid only in the secondary hot water backup function. AC immersion heaters, on the other hand, need alternating voltage. The electricity from the roof must first be converted with an inverter. These solutions are connected accordingly to the normal AC domestic installation.

As a rule, however, most photovoltaic systems are equipped with such inverters, since other electricity consumers in the house also require AC voltage. AC immersion heaters can also be operated with electricity from the grid if required. The power of an immersion heater shows whether the connection must be single-phase (230 volts) or three-phase (3x230 volts).


How does an immersion heater work?

An immersion heater uses electricity in the heat storage to bring the water temperature to desired level by inserting it into the water tank. There it basically works like a simple immersion heater. The heater consists of an insulated electrical conductor that converts electricity into heat.

In order to be able to use an AC photovoltaic heating element, smart energy management is required. In other words, it is about serving all consumers in the house and prioritizing the supply of photovoltaic electricity. First and foremost are the active consumers in the household. The rest of the solar power can then (if available) go to a power storage unit and/or to the heating element. Feeding into the grid is always the last form of application.

For effective energy management, one must bear in mind that the heat requirement itself also fluctuates. When the water in the storage tank has reached the desired temperature, the photovoltaic immersion heater is no longer allowed to heat and can therefore no longer use solar power. However, if warm water is used somewhere in the household, cold water flows into the water reservoir and the immersion heater can start working again.


How much does a photovoltaic immersion heater cost?

Not all immersion heaters are the same: Therefore, the price range of the devices available on the market is from a few 100 to more than 1000 euros. The prices are based on the different parameters that we have already listed in this article. For the combination with a photovoltaic system, a linearly controlled immersion heater is required – these are, for example, our ELWA and AC ELWA-E.

Whichever version you choose, a photovoltaic immersion heater is always a good decision from an ecological and economic point of view. The investment costs are comparatively low and the possible savings through increased self-consumption are quite considerable.


Advantages of a photovoltaic immersion heater for hot water generation

In summary, the following advantages can be stated for an immersion heater operated with photovoltaic electricity:

  • You increase the self-consumption of your photovoltaic system

  • They cover your heat requirements during the summer months

  • You reduce operating costs and also lower your CO2 emissions

  • Require little maintenance

  • Installation and retrofitting are simple and straightforward

  • They have low acquisition costs

  • They represent a sustainable future investment in your home technology.

  • You use renewable energy to generate heat and thus increase your personal independence

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