How to connect panels in the junction box, parallel or serial?

It sounds like you currently have a series wired string of panels. Depending on what your inverter is rated should direct you to how additional panels are wired to it.

While I do not believe the new panels should be wired in parallel I really need more information about your equipment such as how many existing panels, the panel Voc, the inverter max DC voltage input and the specs of your additional panels.

I have SolarEdge SE10000A-US. The system rate to 10.2kW. Currently I have two strings 39 panels

String 1.1: 17 panels @ 300w
String 1.2: 17 panels @ 300w, 2 @ 285w and 3 @250w

The max kW I am reaching now is about 8.5. The image below is about at the max

2020-05-04_12-23-41.png

Here is the view of the entire system. Disregard the 5 new panels, (these were added recently to string 2 - 2 @ 285w and 3 @250w).

solarpanellayout_wires.jpg

I want to add 5 more 300w panels to string 1.1 and connect them to the junction box located next to the panel 1.1.5.

Since the max I produce now is 8.5kW, I should be OK adding another 5 or even 6 panels. The panels will never reach the max wastage. The best placed 300w panels I have now produce reach about 270w.

Per Solar edge the string may contain 25 panels the most for single phase inverter (https://www.solaredge.com/sites/defa..._sizing_na.pdf).

If I get the 300w panels the Voc is 39.76, if I get the 305w panels the Voc is 40.05.

Based on that drawing it looks like there are 19 panels on String 1.1 and 20 panels on String 1.2 so I presume the drawing is not up to date based on what you posted.

I am also assuming the String 1.2 with the 22 panels (that you mention) does not exceed the inverter input maximum. So I would say you can add the 5 (300w - 305w) panels wired in series to the existing 17 panels on String 1.1 to get 22 panels.

Again if the 22 panels wired in series does not exceed the max input voltage for a string then you should be ok.

Here is the update image.

2020-05-04_14-57-45.png

So, if I connect the new panels in series, do I connect it to the red or the black wire? Also does polarity matter?

Yes polarity matters.

How you add the panels depends on how the existing ones are wired. I am not sure how you added the 5 panels on string 1.2 but you will have to similarly wire the 5 new panels in series and then somehow break the existing string 1.1 wiring so the new ones are in series with the existing ones.

The 5 panels that were added to the string 1.2 are connected to panel 1.2.20. I unplugged one of the wire that were coming from the optimizer on panel 1.2.20 and connected the leading wires from the 5 panels.

Somehow find the positive wire at the end of the string and connect it to the negative wire of the 5 new panels. Then connect the negative wire from the existing string and connect it to the positive wire of the 5 new panels.