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I have a sailboat that currently has 2 100 W panels, in parallel, connected to Newpowa 30 A MPPT controller, connected to 580 Ah of 12 V lead acid batteries. Basically runs the fridge. Works well up north. I'm heading south and think I need to up the system to 400 W by adding 2 more 100w panels.

Should I duplicate the original 2-panel system? or combine all 4 panels into 1 controller? or use series for second 2-panel system? or use 4 controllers 1 for each panel?

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Depends on the controller type.

If you have an MPPT controller, the rule is that panel series voltage needs to be higher than battery voltage. Also there may be max panel series voltage. An example: if your battery bank is 12 V, an MPPT controller will always work with a 12 V panel series, and it may or may not work with a 24 V, 48 V or 96 V panel series. Look at the MPPT controller specs to see if it does work.

Panels can always be paralleled. So if you have 12 V battery, 24 V panel series and MPPT controller, and the MPPT controller doesn't support 48 V panel series, then your only option is to parallel the new panel series with the existing 24 V panel series. You get the same voltage but more current. But if the MPPT controller supports 48 V panel series, then you can expand the series for a higher voltage.

However, MPPT controllers may have a maximum current. If you exceed that, you may need to purchase a completely new controller. Controllers can be paralleled so it's possible to have two 100W panels on one controller, two 100W panels on another controller, and the controllers paralleled.

PWM controllers are different: you really should match panel series voltage to battery voltage. So if you have a 12 V battery, you must use only panel series that are intended for 12 V battery (those 12 V panel series have bit over ~20 V open circuit voltage).

PWM controllers also may have a current limitation so you may not be able to parallel infinitely many panels on the same controller. But you can always parallel PWM controllers, so for example you could have two panel series for 12 V batteries, two PWM controllers (one for each panel series), and parallel those PWM controllers at the battery.

Whatever works and is cheap enough is the best choice.

Also each panel in a series should optimally have the same shading and orientation. If you have different orientations for different panels, don't put them into same series.

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If you look at the datasheet of your MPPT solar controller, it will tell you how much Wattage it is able to handle. If this is the controller you are using (https://www.newpowa.com/products/30a-mppt-charge-contoller), the Max PV input power is 390W at 12V. You can safely plug your 4x100W panels in parallel on your controller. If you are ever in light conditions that actually allow your panels to produce 400W all together (rarely happens on a boat), your controller will discard the last 10W with no damage.

If space is a problem, you could replace the panels by two 200W ones.

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