Solar panels are becoming quite popular on boats nowadays, notably on sail boats. However, one question appears to be unanswered and keeps surfacing on forums.
Should panels be connected in series or in parallel? The answer is complicated and depends on many factors. Each case has its own ideal solution; hence, the never ending debates on discussion boards. Below we compare the two approaches and list factors that you might want to consider for your own solar panel system.
When connecting panels in series:
- The first thing is to make sure the current rating for both panels is the same. The current passing through every panel connected in series will be the same; therefore, the panel with the lowest current limits any higher current panels. For example, if we connect one 5 Amp and one 10 Amp panel in series, the overall current passing through both will be 5A reducing the power produced by the 10A panel by about half (the I-V curve of a solar panel is non-linear).
- The second thing you should think about is that at least one of your panels will often be partially or completely shaded. Shadowing can have two important consequences. First, if an entire column of cells is shaded on one of the panels, the current will not be able to pass through the shaded cells and therefore none of the panels will produce power. The second consequence is hot-spot heating, which is local heating around the shaded cells, created by large reverse biased currents from the non-shaded cells. This undesirable effect is destructive to the panel. However, both the consequences listed above can be avoided if the panel has bypassing diodes. The diodes allow current to flow around shaded cells at the cost of a small voltage drop (around 0.7V).
When connecting panels in parallel:
- When connecting panels in parallel, voltage ratings must be the same to keep the performance as high as possible. - Although the hot-spot problem is still present on each panel, shading is not as much of an issue, since a panel could still produce its full power even if another panel is fully shaded.
- The real drawback with paralleling panels is that the combined currents produced by each panel can result in high voltage drops in wires, especially if the distance between the solar panels and the batteries is long. It is thus necessary to use larger wires to keep the voltage drops to a minimum. Larger currents might also require bigger and more expensive charge controllers.
To conclude, there is no single solution that caters to everybody. If needed, a combination of series-parallel setup is possible. If you are not sure about how much shade your panels will receive, it is better to follow the parallel approach. Shading must be avoided as much as possible in order to utilize your solar panels to the fullest.