Photovoltaics – power from the sun

With a photovoltaic system, sunlight can be converted into electrical energy. This is based on the "photo effect", which was discovered back in the 19th century, but could only be physically explained and put to use in the course of the 20th century. The first area of application was space travel. Today, the technology can be found on many a roof and supplies residents with electricity from free solar energy. Photovoltaics are also suitable for reducing the energy costs of commercial and local authority users, while at the same time contributing to climate protection. 

How do photovoltaics work?

The way photovoltaics work is based on solar cells. These cells convert radiant energy into electrical energy. The cells can be connected together to form modules and then installed on roofs, façades and in open spaces. Electricity generation can be broadly divided into two steps:

  1. Harvesting energy: When light falls on the photovoltaic modules (PV modules), electrons are released inside the solar cells. Positive and negative charge carriers collect at the respective electrical contacts, resulting in a DC current flow between the front and back of the cell. This photoelectric effect is created without mechanical or chemical reactions, and so is maintenance-free and not subject to wear.
  2. Power conversion: The DC power generated by the solar generator is then converted by the inverter (commonly also known as a solar inverter or mains feed-in device) into AC power suitable for the grid (230 or 400 volt AC current at 50 Hz). Proven safety standards and fully developed processors, as well as cutting edge power electronics, ensure effective conversion of the solar power. The alternating current generated can then be used in the home or fed into the public grid.

    The efficiency of photovoltaics depends on the manufacturing process of the solar cells. In the Vitovolt  300 from Viessmann, we draw a distinction between monocrystalline and polycrystalline cells. The following table shows what distinguishes the two types.


    Type of solar cellDescriptionEfficiency

    Monocrystalline cells

    Powerful cells from pure single crystals

    14  to over  19  percent

    Polycrystalline cells

    Manufactured from cast silicon blocks with crystals of different orientation

    12  to over  17  percent

    In the Vitovolt 300 monocrystalline photovoltaic modules, particularly dark monocrystalline solar cells are located under a low-iron, highly transparent special glass plate. Together with a black anodised frame and a black Tedlar foil under the cells, this creates modules that offer the highest performance values with maximum stability and a modern design. On these modules, we provide an extended product guarantee for 10  years and a performance guarantee of up to 25  years on at least 80  percent of the rated output. Both monocrystalline and polycrystalline Viessmann solar cells are suitable for use in residential, local authority and commercial buildings.  

    Take advantage of the benefits of Viessmann photovoltaic systems

    With energy costs constantly rising, a photovoltaic system helps users to save money and reduces their reliance on power supply utilities. The power they generate can be used for their own needs or exported to the public grid. Thanks to statutory remuneration and savings resulting from self-consumption, the investment pays for itself in just a few years. A photovoltaic system also increases the value of the property.

    By installing PV modules, users demonstrate their responsible attitude towards the environment, and make an active contribution towards protecting the climate by reducing CO₂ emissions. 8.5  m2 of photovoltaic surface is enough to meet the power demand of the average person.

    Our high quality photovoltaic modules ensure economic efficiency and a long service life. Comprehensive services –– from engineering and sizing through to delivery and maintenance –– round off the photovoltaic range available from Viessmann's professional partners.

    Viessmann photovoltaic technology offers you perfectly coordinated components consisting of PV modules, inverters and installation systems, as well as power storage systems and heat pumps to increase self-consumption.

    Feed-in remuneration and self-consumption

    There are currently two ways in which the solar power generated by a rooftop photovoltaic system can be used: it can either all be exported to the grid, or it can be partially or fully used on site. In the past it was financially more attractive to export all solar power generated into the grid [in Germany]. However, the combination of reduced feed-in remuneration rates and rising energy costs means that self-consumption is becoming ever more interesting. Electricity costs per kilowatt hour are generally far higher than the feed-in tariff for the same kilowatt hour of solar power exported into the grid. Therefore, the photovoltaic power generated is increasingly used on site or temporarily stored in batteries, and only surplus electricity is exported to the grid. The latter is regulated in the Renewable Energy Sources Act (EEG) [Germany]. Under the Act, power supply utilities must purchase any electricity that is not self-consumed and incorporated it into their grid.

    Ensure efficient self-consumption

    Self-consumption offers financial advantages as solar power generated with photovoltaics is cheaper than power drawn from the grid. An optimised system concept with perfectly matched components ensures a high level of self-consumption.

    [1] Photovoltaic system [2] Photovoltaic inverter [3] Photovoltaic meter [4] Consumer [5] Heat pump meter [6] Heat pump [7] Consumption and export/generation meter [8] Public grid

    Complete solution Combining a photovoltaic system with a heat pump

    The most efficient way to generate heat from power is by using a heat pump. With a heat pump, one kilowatt-hour of electricity can provide up to four kilowatt-hours of heat by using free environmental energy. If a heat pump is used to cover the energy demand for heating rooms and domestic hot water, low cost solar electricity enables an inexpensive heat supply. Those intending to combine a photovoltaic system with a heat pump should select environmental heating that specifically optimises self-consumption and can be adapted to match the generating patterns of the PV system. For this purpose, Viessmann has developed a carefully matched system comprising photovoltaics and heat pump.

    Photovoltaic system with Vitocharge power storage unit

    Assuming that it has been installed on a sufficiently large surface area, the photovoltaic system generates enough power during the day to cover the demand of a detached house. However, this fluctuating power supply is faced with varying consumption peaks – for example when the dishwasher, washing machine or tumble dryer are running. And the heat pump, of course, which requires more power for its circulation pump during the heating season.

    A power storage unit balances out these peaks by providing the additional power required from its batteries exactly when it is needed. The Vitocharge VX3 is Viessmann's new generation PV power storage unit, which allows you to increase your self-consumption as well as the efficiency of the entire system. The system will charge the power storage unit when your home is not calling for electricity. This electricity is then used when needed. If the power storage unit is fully charged and no consumers are connected, excess power will be exported to the grid and remunerated accordingly.

    With this system, the annual results show a high level of self-sufficiency for a KfW Efficiency House  40. In the example described, the energy costs would be just €86 for heat and power –– for a whole year!

    [1] Photovoltaic modules
    [2] Solar collectors
    [3] Split air source heat pump
    [4] Outdoor unit of the heat pump
    [5] Mechanical ventilation unit
    [6] Power storage unit

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    Proper planning is essential for economic operation

    In order for the technology to work economically and reliably, a few points need to be considered. In addition to a high product and execution quality, it also depends on the right planning. Anyone considering purchasing a photovoltaic system should firstly check with one of our partners whether the prerequisites have been met. 

    The matrix shows an illustration of the suitable package for the available roof area. The intersection of roof height (Dachhöhe) and roof width (Dachbreite) gives the appropriate package (shown).

    Orientation, inclination and shading of the roof surfaces

    South facing roofs without shade are ideal for photovoltaics. With an inclination of 30 to 40 degrees, the sun's rays strike the Vitovolt 300 solar modules at just the right angle, so that the cells generate plenty of electricity. If the inclination is favourable but the orientation is shifted to the east or west, the yield will be 20 percent lower, on average. To compensate for the losses, you would need to purchase a larger photovoltaic system. The Viessmann surface matrix shows what output is possible on your roof. Interested parties can enter the width and height of the roof surface and quickly see how many modules can be installed.

    Check the load bearing capacity of the roof

    Solar modules add a lot of weight to a roof. A structural engineer will quickly determine whether the roof can support the equipment. If renovation work needs to be carried out on the roof structure, homeowners can apply for financing with a loan from the Kreditanstalt für Wiederaufbau (KfW) [or local equivalent]. 

    Standard values for the design in residential buildings

    The Vitovolt 300 packages from Viessmann make selecting the right photovoltaic system particularly straightforward, based on just a few questions. The system that homeowners need to purchase depends on the number of people living in the household and the intended use of the technology. For example, more solar modules are required if the photovoltaics are also to supply a heat pump with electricity. When combined with a fuel cell, on the other hand, fewer modules are needed. This is because, in addition to heat, a fuel cell produces electricity for self-consumption. The following table shows approximate standard values.

    People in household

    Average power consumption per year

    Photovoltaics only

    Photovoltaics and heat pump

    Photovoltaics and fuel cells


    up to approx. 3000  kWh





    up to approx. 3500  kWh





    up to approx. 4500  kWh





    up to 6000  kWh




    from 5

    up to 6500  kWh




    from 5

    from 6500  kWh




    Quick FAQ about photovoltaics

    Which roof shape is suitable for operating a PV system and is regular cleaning necessary? We answer these and other questions below.

    Viessmann photovoltaic modules can be installed quickly and safely on sloped roofs (between 10 and 60 degrees roof pitch) as well as on flat roofs. The only important factor is that the roof in question can safely support the weight of the modules, even in adverse conditions.

    In practice, around 30  percent of a house's total energy needs can be met with photovoltaics. You can achieve higher self-consumption with the aid of a power storage unit, such as the Vitocharge. A share of 70  to 80  percent is realistic. Using an electric car or an e-bike also makes sense.  Self-consumption can also be increased if you are cooling or heating with an air conditioner.

    If you want to commission a photovoltaic system and receive the feed-in tariff, there are a few things that you need to consider.

    1. You must submit an application for grid connection (grid connection request) to your local power supply utility. You can find out who your local power supply utility is by checking your last energy bill. If there is no name, but only a 13-digit code, then enter it on this website.

    2. The grid compatibility test is carried out by the power supply utility. As part of the check, the utility will verify that the local electricity grid is designed for the planned PV system or not. You can usually complete the first two steps with your solar engineer.

    3. Two weeks before commissioning your photovoltaic system, you must register it with the Federal Network Agency [Germany]. More specifically, the system must be registered in the Market Master Data Register of the Federal Network Agency [Germany]. Registration is extremely important and forms the basis for your feed-in tariff. In other words: no registration = no remuneration!

    4. After commissioning, you will receive a "system passport". This includes all technical details for the system, all documents for installation and a commissioning report. The system passport is comparable with a car's logbook and is indispensable, particularly for legal protection.

    If you commission a photovoltaic system and export some of the electricity generated into the local grid, you have to pay taxes. The taxes you are required to pay will depend, among other things, on the size of your system. In principle, the following types of tax may be levied on a PV system:

    • Trade tax
    • Value added tax (VAT)
    • Land transfer tax
    • Income tax

    Most systems of average size achieve such low output that their operators do not need to pay trade tax. The VAT obligation, on the other hand, only applies when more than half of the self-generated electricity is fed into the grid. System operators only have to pay land transfer tax if the following applies:

    • A property is purchased with a system already installed.
    • In the case, this is an "in-roof system" (solar system installed directly into the roof).

    Only the profit (feed-in tariff minus running costs incl. maintenance and repair costs, insurance, etc.) needs to entered on the income tax return. To be clear on what exactly needs to be entered, it is best to consult your tax advisor.

    Modern solar modules are almost self-cleaning due to their inclination and very smooth surface. In addition, wind and rain remove most of the dust. In the case of snow or coarser soiling caused by fallen branches, for example, cleaning is nevertheless advisable. It is best to hire a specialist company for this work, as the working height poses a certain risk. There is also a risk of the warranty becoming void due to incorrect cleaning.

    If you feed electricity into the public grid, you will receive a payment from your power supply utility for 20  years. Once this period has elapsed, the feed-in tariff will no longer apply. You can still "offer" the self-generated electricity on the market, however. However, depending on the quantity, that search could prove difficult. Alternatively, you can use all of the electricity on site. Another option is to modernise the system. You replace the old modules with new, more powerful ones. You can usually continue using the existing mounting frame.

    Photovoltaic charging station at the Allendorf site

    Viessmann customers, suppliers and guests now have the opportunity to recharge the batteries of their electric vehicles ready for the rest of their journey. In front of the information centre at the Allendorf plant; six "pumps" are available for this purpose. 100  percent of the electricity comes from the Viessmann photovoltaic modules on the roof of the charging station.  

    A BMW i3 painted in bright Vitorange is also charged there. It complements the Allendorf vehicle fleet and is used for journeys in and around the site.