Should I connect my solar panels in series or in parallel?

The ideal way to configure your solar array will depend on a variety of different factors, including your solar controller type, array size, budget, and more. Before delving into each of these topics, it is important to begin by covering the basics of how series and parallel connections impact solar output.

Parallel Connection
When connecting your solar panels in parallel, you will be adding together their current ratings. For example, if you connect two ENERDRIVE | DOMETIC 180W panels (9.1A, 19.8V) together in parallel, you would get an array that produces 18.2A at 19.8V. Adding a third panel to the array would make it 27.3A at 19.8V and so on.

Series connection
When connecting your solar panels in series, you will be adding together their voltage ratings. For example, if you connect two ENERDRIVE | DOMETIC panels (9.1A, 19.8V) together in parallel, you would get an array that produces 9.1A at 39.6V. Adding a third panel to the array would make it 9.1A at 59.4V and so on.

Parallel and Series Connection combined
You can also connect your solar panels in a combination of series and parallel. In this instance you will add together the voltage of each panel in a string, and then add together the current of each string in the array. For example, if you have four ENERDRIVE | DOMETIC 180W panels connected in two strings of 2 panels, you would add together the voltage of each panel in the string (19.8V+19.8V=39.6V) and then add together the current of each string (9.1A+9.1A=18.2A). To get the total power of this array, you would then multiply the array voltage of 39.6V by the array current of 18.2A to get a total of 720w.

As you can see from the examples above, identical panels operating under ideal conditions will produce the same amount of power regardless of which way they have been connected. The decision whether to wire panels in series or parallel starts to become more complicated when real world conditions are taken into consideration. Factors such as solar controllers, mixing panels of different sizes, shading, and cable sizing will also help determine how you configure your solar array.

Solar Controller

Your solar controller will be a limiting factor when determining how to connect your solar panels. If you are using a PWM solar controller, then you will need to match the nominal voltage of your solar panels to the nominal voltage of your battery. If you are using a 12V system, this means that connecting solar panels in series will not be an option and you will be unable to include 24V or residential grid connect panels in your system. If you are using a 24V system, then you will need to connect two 12V panels in series or use 24V panels, and residential grid connect panels will still not be an option.

If you have an MPPT solar controller, then you will be restricted by the unit's maximum input voltage. Consult the specifications on the side of the solar controller or in the owner's manual to confirm the maximum input voltage for your solar controller. While MPPT solar controllers allow for greater flexibility when configuring a solar array, they tend to be more expensive than PWM solar controllers. If your solar array is small or your working on a tight budget, the extra expensive of adding an MPPT controller just to arrange panels in series might not be worthwhile. For more information on PWM and MPPT solar controllers, click here.

Mixing Solar Panel Sizes

In a perfect world, all solar panels in system would be identical in size and produced by the same manufacturer. Unfortunately, this is not usually the case. Solar panels of different sizes and made by different manufacturers are often mixed together based on budget restraints or the availability of roof space on a caravan.

When mixing different solar panels together, determining how much power will be produced becomes slightly more complicated. When calculating the output of different sized panels connected in series, you will need to apply current rating of the lowest panel in the string to all of the panels. When calculating the output of different sized panels connected in parallel, you will need to apply the voltage of the lowest panel to all other connected panels.

To understand what this looks like, we can use the example of two 180W ENERDRIVE | DOMETIC panels from above and add an extra ENERDRIVE | DOMETIC80W panel to the system.  If we connect the panels in series, we would add together the voltages of each panel (19.8V + 19.8V + 17.6V = 57.2V) and multiply by the lowest current rating (4.6A) to get a total of 263W (see example below). This would be an extremely inefficient way of connecting these panels. By adding the 80W panel in series, we dragged down the current rating of the other two panels and actually ended up producing less power than before.

If we connect the same panels in parallel, we would add together the current ratings of each panel (9.1A + 9.1A + 4.6A = 22.8A) and multiply by the lowest voltage rating (17.6V) to get a total of 401W of power (see example below). This is a significant improvement over the series wiring configuration, but we still are still producing only 401W instead of the 440W of total output our panels are capable of. Even though we added an 80W panel to the system, we only gained 41W of additional power.

While parallel wiring is the far more efficient choice in the above examples, this may not always be the case. It is important to check the specifications of all of your panels before deciding how to wire them. If your panels are wired in series, then there is good chance that adding a single panel of a different size will do more harm than good to your solar production.

Cable sizing

One of the biggest advantages of connecting your solar panels in parallel is a reduction in the size and length of cable that you need to run. Connecting solar panels in series drastically reduces the amount of cabling that you need to run because you are only connecting one positive and one negative cable in the string to the solar controller. All other solar panels will be connected directly to each other, which can often be done without purchasing any additional extensions or connectors in between. Adding more panels in series will also not require you to run a wider diameter cable because it is only the voltage of the array that is increasing, not the current.

If you are connecting your panels in parallel, then the positive and negative leads of the solar panel would need to be wired to either a distribution point or all the way back to the solar controller. If you are connecting your panels to a single distribution point, then the cable between the distribution point and the solar controller will need to be wide enough in diameter to accommodate the combined current ratings of your solar panels. Depending on the size of your solar array, this extra cabling can add significant expense and weight to your electrical system.

Shading

While series connections have advantage when it comes to cabling, parallel configurations have an edge when it comes to dealing with shading. The power of a solar panel drops significantly when it any part of it is exposed to shade. In a parallel array, it is only the panel that is directly affected by the shading that will see a reduction in output. The other panels in a parallel array will continue to function normally. When a panel connected in a series array is exposed to shade, the entire array will be affected and overall output will drop significantly.

Summary

There is a lot to consider when deciding how to wire your solar array. ENERDRIVE | DOMETIC generally recommends wiring solar panels in parallel for most setups because shading is such a significant obstacle to solar production in caravan and 4WD applications. Finding a sunny spot in a crowded campground is easier said than done and there is a good chance that shadows will shutdown your entire array unless you are willing to move your vehicle throughout the day. Parallel setups also work well with small arrays and allow for the use of budget friendly PWM controllers.

For larger solar arrays, where exclusively parallel solar arrays are impractical, wiring your panels in series and parallel can help you reduce the amount of cabling you need to run while also reducing your systems vulnerability to shading.