Series or Parallel? The Electrical Decisions That Define PV System Performance
Behind every solar photovoltaic system—whether a small rooftop array or a multi-megawatt plant—lies a fundamental electrical design choice: how PV modules are connected. Figure illustrates the conceptual difference between series and parallel module configurations, a topic that appears simple on the surface but has far-reaching implications for system voltage, current, efficiency, safety, and reliability.
At the most basic level, PV modules generate direct current (DC) electricity. Each module produces a specific voltage and current under standard test conditions. To meet the requirements of inverters, batteries, and grid interconnection standards, individual modules must be electrically combined. This is achieved through series and parallel connections, or a combination of both.
In a series connection, the positive terminal of one module is connected to the negative terminal of the next. The result is that voltages add up, while the current remains the same as that of a single module. For example, if one module produces 40 V at 10 A, connecting ten modules in series results in a string voltage of 400 V at 10 A. Series connections are therefore primarily used to increase system voltage.
Higher voltage operation offers several advantages. For a given power level, higher voltage means lower current, which reduces resistive losses in cables. This allows for smaller conductor sizes, lower material costs, and improved system efficiency—particularly important in rooftop and utility-scale installations with long cable runs. As a result, most grid-connected PV systems rely heavily on series-connected strings to match inverter DC input voltage ranges.
In contrast, a parallel connection involves connecting all positive terminals together and all negative terminals together. In this configuration, currents add up, while voltage remains constant. Using the same module example, two parallel-connected modules would produce 40 V at 20 A. Parallel connections are used when higher current is required without increasing voltage.
Parallel configurations are common in systems where voltage must be limited, such as low-voltage battery-based systems or certain off-grid applications. However, higher current operation introduces its own challenges. Increased current leads to higher resistive losses, thicker cables, and greater thermal stress on connectors and protection devices. For this reason, extensive parallelization is generally avoided in modern grid-connected PV systems.
In practice, most PV arrays use a series–parallel combination. Modules are first connected in series to form strings that meet inverter voltage requirements. Multiple strings are then connected in parallel to increase total power output. This approach balances voltage optimization with scalable power capacity.
Figure also highlights an important operational consideration: mismatch effects. In series-connected strings, the current is limited by the weakest module. Shading, soiling, or degradation affecting a single module can reduce the output of the entire string. This characteristic has driven the adoption of bypass diodes, module-level power electronics, and careful string design to minimize mismatch losses.
Parallel connections are more tolerant of individual module variations, but they require robust protection. Each parallel string must be protected by fuses or circuit breakers to prevent reverse currents under fault conditions. Improperly designed parallel systems can pose serious safety risks, including overheating and fire hazards.
From a technician’s perspective, understanding series and parallel behavior is essential for troubleshooting. Abnormal string voltages may indicate open-circuit faults in series connections, while unexpected current imbalances can signal issues in parallel strings. Accurate diagnosis depends on a clear grasp of how electrical quantities behave in each configuration.
Design standards and regulations also reflect these principles. Maximum system voltages are defined to ensure insulation integrity and personnel safety, while current ratings influence cable sizing and protection coordination. Modern PV codes are built around the predictable electrical behavior illustrated in Figure.
Ultimately, series and parallel configurations are not merely wiring choices—they are strategic design decisions. They influence system efficiency, cost, safety, expandability, and long-term performance. Mastery of these fundamentals separates robust PV system design from marginal installations.
Figure distills this critical concept into a visual form, reminding us that even the most advanced solar technologies rely on sound electrical fundamentals. As PV systems continue to scale and integrate with smart grids and storage, these basic principles remain as relevant as ever.
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