Table of contents:

1. Optimal dimensioning of photovoltaic modules
2. Recommended number of photovoltaic modules per product
3. Selecting the right photovoltaic modules
4. Shading and the optimal orientation of the photovoltaic modules
5. Optimal cable length and dimensioning

The correct selection of photovoltaic modules and the optimal cable dimensioning

Most standard photovoltaic modules are compatible with fothermo products. To improve heating performance, some basic aspects should be considered: choosing the right number of photovoltaic modules, optimal orientation for maximum solar radiation and keeping cable runs as short as possible to minimize power losses.

The optimal dimensioning of photovoltaic modules

The optimal dimensioning of the photovoltaic modules is crucial for efficient hot water preparation with fothermo products. The required photovoltaic power depends on the volume of water to be heated and the output of the respective solar boiler or solar heating rod.

Recommended number of photovoltaic modules per product

• 10 liter solar off-grid boiler: 1 photovoltaic module
• 30 liter solar off-grid boiler or hybrid boiler: 2 photovoltaic modules
• 80 liter solar off-grid boiler or hybrid boiler: 3 photovoltaic modules
• 200 liter solar storage tank: 6 photovoltaic modules
• 300 liters of solar storage: 6 photovoltaic modules
• 550 W solar heating element: 2 photovoltaic modules

If a fothermo product is used in combination with the switching device, we recommend connecting 4 photovoltaic modules to the switching device. This enables optimal use of the solar power generated.

Depending on space and personal preferences, the number of photovoltaic modules per product can also be reduced or increased. This offers flexibility in adapting to individual needs.

The output of the photovoltaic modules is usually greater than the heating output of the fothermo products. This deliberate oversizing ensures that there is enough solar energy available to generate hot water even on cloudy days. The additional capacity compensates for fluctuations in solar radiation and ensures a reliable hot water supply at any time of year.

Choosing the right photovoltaic modules

It is crucial that the open circuit voltage (Voc) of the photovoltaic modules remains below the maximum open circuit voltage of the fothermo product. Depending on the model, this is 42.4 V or 50 V.

Nevertheless, it is important to ensure that the open circuit voltage of the photovoltaic modules is as close as possible to the maximum open circuit voltage of the fothermo product. This is crucial because the maximum heating output of the fothermo products depends on the voltage of the photovoltaic modules.

As a rule of thumb, we recommend that the open circuit voltage of the photovoltaic modules used is no more than 3 V below the maximum open circuit voltage of the respective fothermo product. This ensures optimal energy yield and efficient hot water preparation.

Example : The maximum open circuit voltage specified in the data sheet of the solar heating rod is 50 V. Therefore, the photovoltaic modules connected to the solar heating rod should ideally have a maximum open circuit voltage of 47 V to 50 V. It is not possible to connect photovoltaic modules with an open circuit voltage (Voc) of more than 50 V. This would lead to a defect.

Shading and the optimal orientation of the photovoltaic modules

Since the photovoltaic modules in fothermo products are connected in parallel , they are less susceptible to performance losses due to partial shading. The performance of the unshaded modules is maintained.

Optimal alignment:

• A south-facing orientation offers the highest energy yield.
• An east-west orientation can enable more consistent electricity production throughout the day.
• Modules should be mounted towards the sun to minimize reflection losses.
• The optimal angle of inclination of the modules should be adjusted according to the latitude. In Central European regions, this is usually between 30 and 35 degrees.

Another aspect is the regular maintenance of the modules. Contaminants such as dust, leaves or bird droppings can affect performance, which is why regular cleaning is recommended.

The optimal cable length and dimensioning

The distance between the photovoltaic modules and the fothermo product should not exceed a maximum length of 25 meters, otherwise power losses increase disproportionately. This results in a maximum cable length for the forward and return current of 2x 25 meters = 50 meters

Recommended cable dimensioning:

• Up to 25 meters cable length: High-quality solar cables with a cross-section of 6 mm² .
• This minimizes energy losses and ensures safe power transmission.

A larger cable cross-section can be useful, especially for very long cable runs, to further reduce voltage losses. In addition, weatherproof insulation should be used to ensure the longevity of the cable. It is important to ensure that the solar connectors are crimped to the cable to a high quality in order to

Interesting to know: Cable losses play a minor role in the fothermo system. High losses only occur when there is a lot of power available. On sunny days there is enough energy available to heat the water anyway. Losses of 2 to 5 percent are then acceptable. In the winter months, when every available amount of energy should be used to generate hot water, the line losses are lower due to the lower photovoltaic power.

Recommended next steps:

1. Check the number of recommended photovoltaic modules for your preferred fothermo product.
2. Check the distance between the photovoltaic module and the boiler. This should not exceed 25 meters.
3. Selection of the right photovoltaic modules with suitable open circuit voltage.
4. Installation and commissioning of the fothermo photovoltaic system