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# Mismatched panels & MPPT sizing

Discuss Mismatched panels & MPPT sizing in the Solar PV Forum | Solar Panels Forum area at ElectriciansForums.net

#### opfrei

##### EF
DIY
Welcome to ElectriciansForums.net - The American Electrical Advice Forum

Hi there,

I just purchased a sailboat and its electrical system was put together in questionable ways..

It came with an 815W array of mismatched PV panels, linked to a Xantrex C40 (40A) PWM charge controller, charging a 1030Ah 12V lithium battery bank.

First question: can mismatched panels damage the controller or reduce array efficiency in any way?

Second, I’d like to replace the PWM controller with a Victron MPPT controller (or the like) to boost efficiency... how do I calculate the total voltage/amp rating for the controller given the differing panel specs? Do I simply add the VMP/IMP/etc values together? I guess this would also depend on whether I wire the panels in series or in parallel... suggestions?

The run from panel to controller is relatively short, but would it still be best to wire the panels in series (to 96V) then have the controller step it down to 12V?

The panel specs are as follows:
3x Sun Electronics 95W
(VOC21.2V, VMP17.4V, IMP5.46A, ISC5.99Am)
3x Sun Electronics 90W
(VOC21.2V, VMP17.5V, IMP4.57A, ISC5.6Am)
2x Sun Electronics 130W
(VOC24.1V, VMP19.8, IMP6.55A, ISC7.11Am)

#### mvas-sti

Wiring all 8 pv panels in series ...
Add the Vmp volts but then LIMIT the entire circuit to the lowest Imp Amps = 4.57 amps
145 Vmp volts = 52.2 V + 52.5 V + 39.8 V
663 Watts = 145 volts x 4.57 amps

Wiring all 8 panels in parallel ...
Add the Imp amps and average the Vmp volts ...
43 Imp Amps = 16.4 A + 13.7 A + 13.1 A
752 Watts = 17.5 Volts x 43 Amps

The above Watts values are for comparison purposes, not actual production.
Assuming my math is correct, parallel is better

If series connected, then what model # Disconnect Switch could you use for 176 Voc Volts DC ?

I am not sure how you computed "Series connected = 96 Volts".

Last edited:

#### opfrei

##### EF
DIY
Thanks for your response! Please disregard my initial math and let me know if this sounds anything close to correct:

When connecting these 8 panels in parallel they should output:

Total watts 815W
Average VOC 21.9
Average VMP 18
Total IMP 43.2A
Total ISC 49A

According to the Victron calculator I should choose their 150V / 60A controller.. does this seem right to you?

Also consider I will be taking these panels to both temperature extremes with variable shade (on a sailboat)...

#### Zerax

##### -
I also have a boat... with solar panels and MPPT controllers. My advice to you would be to have 3 separate controllers, one for each 'group' of panels. The enemy of solar is shadow and shade... so multiple controllers will help give max output if there's a shade issue (like your mast/rigging etc.). Furthermore, it provides for resilience... the marine environment is a harsh one and should a single large MPPT controller fail, you'll have no power and the repair issue to deal with (if you're a few hundred miles offshore ??)

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