Due to their size, solar photovoltaic arrays are easily visible when deployed in residential and commercial buildings. However, since the underside of solar panels is hidden from sight, you may wonder exactly how individual modules are connected together.
The most common way to connect solar modules is called a series connection in electrical engineering. Basically, solar modules are wired end-to-end, similar to how train wagons are attached with hooks at their ends. A group of solar modules wired together in series is called a string circuit. In turn, these string circuits are connected to inverters, which condition their power output to be usable by home appliances and other electrical equipment.
Solar photovoltaic arrays used in homes and small businesses are often connected in one or two string circuits, due to the small number of solar modules. As the scale of the project increases, more string circuits are added to accommodate a larger number of modules. Note that many inverter models are designed to manage the input of multiple string circuits, which means there is no need to budget one inverter per circuit.
The main advantage of the series connection is simplicity: a string circuit is just a group of solar modules wired together in a chain, with conductors designed especially for that purpose. Thanks to the simplicity of series connections, you can expand solar array capacity in the future by simply adding more string circuits and inverters.
Of course, the main disadvantage of series connections is that all solar panels become interdependent:
Solar panels connected in a string circuit operate better when their energy production profile is similar. As an example, assume you have a dual-pitched roof, where one side faces east and the other faces west:
A qualified solar installation company can determine the best layout and circuit arrangement for a photovoltaic array, depending on the site conditions found in your property. Productivity can be enhanced by optimising the orientation of your array, and by avoiding areas that are frequently covered with shadows.
Unlike a series connection, a parallel system wires electrical devices between the same two contact points, instead of using a chain-like configuration. A series connection adds the voltage of solar panels under a shared current, but a parallel connection adds current under a shared voltage.
The cost of electrical equipment is more sensitive to high current, so it makes sense to have a higher voltage and a lower current. For example, conductors are sized based on the current they carry, and a solar array connected in parallel would require very large and expensive wiring.
The electrical protection devices and inverters also become much more expensive with a high current output. To illustrate this concept, assume you have 10 solar panels, each having a voltage of 30V and a current of 10A.
Also keep in mind that commercial string inverters and solar modules are designed for a series connection. A parallel system would probably require an improvised configuration, which is less reliable and more dangerous.
Microinverters are devices that can be attached directly to solar panels, converting the power output from direct to alternating current. In the example above, microinverters could be used to condition the 30V 10A output of solar panels, delivering an AC output of 230V and a current of slightly over 1A. Thanks to the reduced current output per module, a parallel connection becomes feasible with microinverters.
The main drawback of solar arrays with microinverters is a higher cost than conventional arrays with string circuits. They are also more demanding in terms of maintenance, since there is one inverter for each solar module!
The main advantage of a parallel connection is that solar modules become independent from each other. For example, if one module is malfunctioning or covered with a concentrated shadow, no other modules are affected. In addition, some microinverters include monitoring features that allow immediate detection of performance issues.
Power optimisers provide an alternative to microinverters. They use a series connection and string circuits, but each solar module is equipped with a smart electronic unit that manages voltage and current in real time. If there is a performance issue affecting one module, the power optimiser adjusts the output so that other modules in the circuit are not affected. Like microinverters, power optimisers can also include monitoring features to pinpoint the exact location of issues in a solar array.