PV‑String: How to correctly plan the strings for your solar system

Planning a photovoltaic system doesn't start with the inverter and doesn't end with the number of modules. String planning determines whether your system runs efficiently or is wasted untapped potential. Here you can find out what is behind the so-called PV string and why it is more than just a technical detail.

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What is a PV string?

A PV string is a series circuit of several solar modules, which together generate an electrical voltage and are connected to an inverter. All modules in a string work like a circuit. The voltage of the modules is added up while the current remains the same. The correct string planning is crucial for the efficiency and safety of a photovoltaic system.

Why is proper string planning so important?

The right string planning is crucial so that the photovoltaic system works efficiently and does not cause any energy losses. Incorrectly connected or unevenly oriented modules can reduce the performance of the entire string. Inverters, cable cross-section and shading must also be taken into account when planning. Well-thought-out string planning ensures the yield, longevity and operational safety of the PV system.

How many strings per PV system?

The number of strings per PV system depends on the system size, roof orientation and the inverter. Two to four strings are usually used, but even more for larger systems. It is usually useful to have a separate string for each roof area or orientation so that you can avoid loss of yield due to shading or different levels of radiation. The exact number is determined individually during system planning.

How many modules per string?

The number of modules per string depends on the module voltage, the inverter design and the temperature. In practice, a PV string usually comprises 8 to 15 modules. It is important that the total system voltage does not exceed the maximum input voltage of the inverter. Separate strings should be used for different roof orientations to avoid loss of performance.

Inverters and MPPT — how are they related to PV strings?

inverters convert the direct current from the PV strings into usable alternating current. The integrated MPPT (Maximum Power Point Tracker) makes optimal use of every string by continuously setting the ideal voltage and amperage. A separate string should be used for each MPP tracker, especially when there are different orientations or shading. In this way, the plant achieves maximum yields and avoids performance losses.

Why 2 strings per MPPT?

Two strings per MPPT are possible if both have exactly the same electrical properties. This means: the same module types, the same number, the same orientation and the same shading situation. Only then can the MPPT determine the same optimal working point for both strings. If the strings differ, the MPPT cannot find a common point — this leads to loss of revenue. If conditions differ, each string should receive its own MPPT.

Technical aspects of interconnection & cabling

PV modules are connected to strings in series, with the voltages of the individual modules adding up. All modules of a string should be identically constructed and have the same orientation. The cabling is carried out with UV-resistant solar cables, usually with a 4 mm² or 6 mm² cross section. The cable length must be kept as short as possible to avoid losses.

The string lines lead to the generator junction box or directly to the inverter. DC circuit breakers and surge protection are mandatory there. It is important that the maximum system voltage is not exceeded — especially at low outdoor temperatures, as the open-circuit voltage then rises. Faulty cabling can result in loss of power, fire risk or damage to the inverter.

Challenges & sources of error with strings

One of the biggest challenges with PV strings is partial shading. Even one shaded module lowers the performance of the entire string. Different orientations or inclinations within a string also lead to mismatch losses.

Other sources of error include cables that are too long, incorrect cable cross-sections, incompatible module types or insufficiently dimensioned inverters. In addition, exceeding the allowable system voltage when it is cold can damage the system. Faulty sockets, lack of surge protection or poor connection technology can also reduce yields or cause safety risks. Careful planning and professional execution are therefore essential.

Tips for planning and executing strings

Strings should be carefully planned and professionally executed so that your photovoltaic system works efficiently over the long term. The following tips will help you avoid typical mistakes and optimize your yield:

• Select the same orientation and inclination: Modules in a string should be identically oriented and tilted. Differences result in unequal amperages and reduce performance.
• Only connect identical modules: Only use modules with the same power, voltage, and amperage within a string. Mismatching reduces overall efficiency.
• Absolutely avoid shading: Even slight shadows on a module can significantly reduce the yield of the entire string. Therefore, plan strings in shadow-free zones.
• Adjust string length to the inverter: Pay attention to the minimum and maximum voltage of the inverter. Especially at low temperatures, the open-circuit voltage must not be exceeded.
• Use suitable cable cross-sections: The cable cross section must match the cable length and amperage. Cables that are too thin cause losses and can overheat.
• Integrate surge protection and circuit breaker: Each string should be protected to prevent damage in the event of lightning or fault currents. Disconnectors make maintenance and repair easier.
• Documenting and marking strings: Clean documentation of the string structure helps with subsequent troubleshooting and maintenance. Monitoring systems also benefit from clear allocation.

conclusion

The correct planning of PV strings is crucial for the yield, safety and lifetime of a photovoltaic system. Homeowners should pay attention to identical modules, appropriate string lengths and optimal orientation. Shading and mismatching can be avoided by using separate strings and multiple MPPTs. The cabling must also be carried out professionally and in accordance with standards. If you pay attention to these points, you get the most out of your PV system.

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