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A power storage unit as a complement to a photovoltaic system

Viessmann photovoltaic systems can generate enough electricity to supply a family of four for an entire year. The main problem with photovoltaic systems, however, is that they tend to provides electrical energy when most household inhabitants are not at home –– in the afternoon hours, for example. If no connected consumer is active or the connected devices cannot fully consume the electricity, the electricity is fed into the public grid.

In recent years, however, the feed-in tariffs have fallen steadily. At the same time, the average electricity price has been rising. So, there's a lot to be said for increasing self-consumption. A power storage system can help. That's why Viessmann has launched the Vitocharge VX3 photovoltaic power storage system. This battery storage system stores the electricity generated during the day and makes it available when it's needed.

How do power storage systems work?

Photovoltaic system

Solar power storage offers the great benefit of being able to use self-generated electricity when it's needed. That means even when the sun is not shining.

Electricity can be stored in two ways –– directly and indirectly. For domestic use, however, only indirect storage systems are an option. Direct storage of electricity takes place using components such as coils and capacitors, and only lasts for seconds. This makes these types unsuitable for detached or two-family houses. They tend to used in industry, primarily for grid stabilisation. So when it comes to photovoltaics with storage, the system usually involves an electrochemical storage unit such as a battery.

The storage process briefly explained

The functional principle is quite simple. The PV battery storage system stores the electrical energy, similar to a rechargeable battery, until a demand arises in the household. It then passes that power on to the connected consumers (light, refrigerator, TV system, etc.). In detail, this means that when the sun's rays hit the photovoltaic modules, they are converted into direct current. But this cannot be used in the home. To remedy this, an inverter is installed in the circuit to convert the direct current into alternating current. The power storage unit comes into play if there are no consumers for this power in the home. Lithium-ion batteries from Viessmann convert electrical energy into chemical energy. If discharge then occurs, this process is reversed.

Important: During the discharge process, the solar power storage unit is never discharged entirely. There is a distinction between "nominal"and "usable" capacity. The difference is the depth of discharge. The Vitocharge VX3 has been developed according to a special operating strategy and is similar to that of an electric vehicle. Ageing of the battery is compensated over the service life with an increase in release of the depth of discharge. In the case of an electric vehicle, this means that the range of the vehicle is kept constant for as long as possible over its service life. Only when the depth of discharge has reached 100 % does the usable capacity become less. If the power storage unit is charged and there are no connected or active consumers, the electricity is fed into the public grid. Remuneration is governed by the Renewable Energy Sources Act (EEG) [Germany].

What are the distinguishing features of Viessmann photovoltaic storage units?

PV storage units close the gap between supply and demand. This helps to increase self-consumption and reduces energy costs. The electricity that is generated can be used much more efficiently. Viessmann power storage systems also ensure greater independence from power supply utilities and relieve strain on the grid.

With our Vitocharge product range, we offer lithium-ion battery storage units with high efficiency and a long service life. Our models have a service life of up to 20  years or a guaranteed energy throughput of 9.6  MWh per 5 kWh battery. On average, you can expect around 250  full cycles per year. Vitocharge  VX3 batteries can be connected in series. This allows for flexible storage sizes and perfect adjustment to your individual electricity needs. In addition, they are suitable for both mains parallel and mains substitution mode. This means that Viessmann power storage units can operate in parallel to the power supply via the public grid, as well as supply the most important consumers in the event of a power failure.  A requirement for grid backup operation is a back-up box – this enables the switchover from grid parallel to backup power operation.

New generation of power storage units: more compact, quiet and powerful

New generation Vitocharge VX3 batteries give homeowners the opportunity to store electricity easily, and thus use it more efficiently. The proven and reliable lithium iron phosphate batteries are designed for a long service life. That is why Viessmann offers a 10  year cash value replacement guarantee on the battery cells. In addition, the system can be easily expanded during the first year of operation if power demand changes. This puts the modular Vitocharge  VX3 power storage unit at the heart of Viessmann's well thought-out energy solutions for heat, power and mobility. From heat generation to the photovoltaic system to the charging station for the electric vehicle, everything comes from a single source. You'll only get that from Viessmann.

Is a power storage unit worthwhile?

With a power storage unit from Viessmann, you get a product that has many uses. The Vitocharge VX3 can be used as a hybrid PV power storage unit, as an AC-coupled power storage unit or as a pure PV inverter. This makes it suitable for use in both new and existing systems.

Power storage units for new PV systems

With the decreasing feed-in tariff, installing systems for self-consumption makes more and more sense. However, it is important that new system is matched to the house and the electricity demand so that it is also economical. To ensure that the environmentally responsible, self-generated electricity always finds a consumer, it pays to have a uniform energy solution in place. Some examples are: 

  • Photovoltaic system + power storage unit + heat pump
  • Photovoltaic system + power storage unit + fuel cell heating appliance
  • Photovoltaic system + power storage unit + electric heating and DHW system 

The energy from the photovoltaic system is stored by the power storage unit and consumed directly by electrical consumers, such as the heat pump. In the event of a surplus of energy, the energy is stored temporarily in the power storage unit battery and released again when required. The home and heat pump are thus largely supplied with power generated sustainably and efficiently on the roof.

With a photovoltaic system and fuel cell heating appliance, you get two power generators that complement each other perfectly. In the summer, the photovoltaic system generates the electricity for the house (and optionally an electric vehicle). In the winter, the fuel cell heating appliance supplies more electricity due to its operating time. This means that your home (and optionally your electric vehicle) can be supplied from your own power resources all year round. This increases self-sufficiency on the one hand, and reduces your electricity costs on the other.

In a fully electric system, the energy generated by the photovoltaic system is used for electric heat generators such as infrared area heating, underfloor heating or domestic hot water heating. In the event of a surplus of energy, the energy is stored temporarily in the power storage unit. A home with both electric space heating and DHW heating can thus achieve a high degree of self-sufficiency and sustainability and enjoy the benefits of infrared heat with low standby consumption.

System operators receive a fixed payment for 20  years for the electricity that they export to the grid. Once the Renewable Energy Sources Act subsidy for your existing photovoltaic system expires, public grid operators are no longer obliged to accept the power you produce. Furthermore, "direct marketing", whereby self-generated power is sold on the electricity exchange, is not necessarily feasible. In this situation, it is worth increasing self-consumption with Viessmann products. The high degree of flexibility of the Viessmann components allows an existing photovoltaic system to be combined with a power storage unit as a system solution in this instance. If necessary, the system can be easily expanded with additional battery modules.  

In addition to the usual household appliances that require electricity, we can also increase self-consumption with an electrically operated Vitocal  262-A DHW heat pump, for example. It is the perfect addition for pleasant DHW convenience. The use of self-generated electricity results in only lower running costs. The electrically operated Vitovent  300-W mechanical ventilation system also boosts our sense of wellbeing and comfort at home, and contributes significantly to preserving the building fabric with fresh, clean air and an optimum humidity level.  

How large does the power storage unit need to be?

Whether for a new system or an existing system, it all depends on the right size of the power storage unit. The power storage unit should have enough capacity to supply a household with solar electricity from the evening until the next morning. The size or storage capacity of a power storage unit depends on both the annual electricity consumption and the rated output of the existing or planned PV system. The following rule can be used as a guide: 1 kWp PV = 1 kWh battery = 1000 kWh household electricity consumption.


Example 1 with heat pump

Household + heat pump: 8000 kWh

PV = 8 kWp


Power storage unit = 8 kWh



Example 2 with heat pump and electric vehicle

Household + heat pump + electric vehicle = 12,000 kWh

PV = 12 kWp


Power storage unit = 12 kWh 


Storage units that are too large or too small are uneconomical

If the power storage unit is oversized, it will store too much electrical energy without it actually being used. In addition to the unnecessarily high additional acquisition costs, you will also suffer losses due to the loss of remuneration for the electricity fed into the grid. By contrast, if the power storage unit is too small, system owners will have to purchase more electricity. This is significantly more expensive than self-generated power from the roof. Consequently, it is essential that you seek advice from one of our trade partners. 

Storing solar energy and managing it efficiently

Digital networking and control of all system components makes it easier to store renewable energies and to harmonize their generation and consumption. In this respect, the Viessmann GridBox is an optimal supplement for system solutions consisting of a PV system and energy storage. This is because the application clearly visualizes energy flows in the house and thus creates full transparency. The basis for this is the energy dashboard. It illustrates, among other things, the output of the photovoltaic system, the charging status of the energy storage unit, and the current power consumption in the house. Daily trends, weekly reports and information on the CO2 balance make it easier for users to maintain an overview at all times. Available optimization functions for the PV system, solar energy storage, hot water heating systems and electric vehicles make the system even more efficient.  

Power storage unit product range

Viessmann power storage units increase your self-consumption of the energy you generate and improve the efficiency of the photovoltaic system. The system will charge the power storage unit when your home is not calling for electricity. This power is used as required, for example to run electrical appliances. If the power storage unit is fully charged and no consumers are connected, excess power will be exported to the grid and remunerated accordingly.

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