Wiring solar panels.

[Allan]

I'm connecting four solar panels to my existing Victron 75/15. Two of the panels are 50w each and two are 45w each. I plan to wire each pair in series then wire the pairs in parallel. This keeps the voltage up to allow me to use smaller wire. I'm wondering if there is anything I'm missing?
Allan

 

[noelex]

What is the Vmp and the location of the panels?

 

[Allan]

What is the Vmp and the location of the panels?

The two 50w panels are quoted at Vmp of 17.6v the 45w panels only have Vmpp of 19.1v quoted.
I'm not quite sure exactly what you mean by the location. They will be used as roving panels on the deck of a sailing vessel with 4m flying leads. Our sailing area is between Sweden and Grenada. Sorry I can't be more specific.
Allan

 

[noelex]

A 1.5v (3v in series) difference is quite large.

Using a single MPPT controller will not cause any damage, but the disparate MPP voltages mean there will be a reasonable loss of total output.

Using two controllers would be better.

 

[Allan]

A 1.5v (3v in series) difference is quite large.

Using a single MPPT controller will not cause any damage, but the disparate MPP voltages mean there will be a reasonable loss of total output.

Using two controllers would be better.

Is there a formula for deciding how much power may be lost? Another way to wire them would be to wire each pair in parallel then series the two sets. I would need to revisit my current and voltage drop calculations.
Allan

 

[noelex]

The exact drop in efficiency is difficult to predict, but a rough estimate can be made by assuming the chosen Vmp will be close to the lower figure which is 17.6v (38.2v in series). Operating the two 45w (90w in series) at this voltage would reduce the nominal wattage of the two 45w to 45x17.6/19.1=41.5w. This is a reasonable loss, but is perhaps acceptable if you don't want maximum efficiency.

However, with the two very disparate MPP voltages controllers have difficulty tracking the correct powerpoint. If the controller locks onto the MPP from the two 45w panels both the 50w panels will be shut down almost completely resulting in a severe loss. This wont happen often, but will result in a very significant loss on occasions.

In short it is long way from ideal.

An alternative would be to wire one 45w panel to a 50w panel in series and then to connect these sets together in parallel. This has its own set of compromises, but I suspect it would better.

Two controllers would still be better solution.

 

[Allan]

I've decided to do all of the wiring, except the panel linking at home. I'll then try the panels, on a sunny day on the boat to see which method gives the best output.
Allan