Mixing solar panels with different ratings

More power please!

A common question we frequently get asked is how to add more power to an established solar system. Often the question is loaded with the desire to add an additional (and bigger) panel to the roof of a motorhome, caravan , campervan, or a boat deck to maximise the energy generation from the available space. When wiring panels in parallel, we always advise matching the new panel to the Voc voltage rating of the existing panel, and this is why!

Testing

On an Autumn morning, our in-house technical team set up a test at the company's headquarters in Bredon, Worcestershire. Conditions weren't perfect for maximum solar generation but perfectly reasonable test conditions.

The purpose of the exercise was to establish what the effect of mixing panels of different wattages and voltages together would be and, if doing so, would this affect the efficiency of either panel, or both? The equipment used to take the readings was the EY800W Solar Panel Multimeter.

Functional test

The panels used in the test were the PV Logic rigid panels - the 60W STP060 and the 155W STPU155. The nominal voltages are reasonably similar but the wattages are not.

The results in the table above show that:

Watts

The maximum number of watts generated theoretically should simply be achieved by adding the values in blue, 60W + 155W = 215W
The actual individual panel readings (shown in red) were those achieved during the test
The number of Watts individually generated by each panel is highlighted in red
The number of Watts shown in bold black (129W) is the amount that both panels generated when wired together in parallel
This is noticeably lower than the 145.03W achieved by simply adding the individual amounts generated by each panel (in red)

Volts

The voltage of the STP060 measured on its own was within spec at 17.42V
The voltage of the STPU155 measured on its own was within spec at 22.89V
The voltage that both panels generated when wired together in parallel was 18.3V which is noticeably closer to that of the STP060 than that of the STPU155

Amps

The amount of current generated individually by each panel is highlighted in green at 2.61A ad 4.33A respectively, which theoretically when added together should make 6.94A available
The amount of current generated by the panels when wired together in parallel was very similar at 7.05A

Conclusions

The voltage of the higher panel is dragged down almost to the voltage of the lower voltage panel, so it is reasonable to think that the larger the disparity between the panel voltages, the less effective the higher voltage panel will be.

The current available from both panels appears to be fully accessible, though with the reduced voltage of the larger panel the total power obtainable from the system overall is reduced.

Consequently whilst mixing panels of different wattages/voltages will not prevent them from working, it will most certainly affect the efficacy of the overall installation.

In simple term, mixing and matching panels wastes power!

Our recommendations

When considering multiple panels in parallel on a single system we recommend keeping all the panels connected to the same controller of the same make and model, if possible. This will minimise any losses caused by mismatched panels.

If this isn't achievable, any additional panels should be as close to the Voc rating of the existing panel as possible.