Extend the output wires of the solar panel the positive wire of the solar panel is terminated with an mc4 female connector marked.
How to tell which wire is positive solar panel.
Wiring solar panels in parallel causes the amperage to increase but the voltage remains the same.
Another way to find the polarity of the solar panel is to check with a volt meter.
When stringing in series the wire from the positive terminal of one solar panel is connected to the negative terminal of the next panel and so on.
With parallel solar panels the positive terminal from one panel is connected to the positive terminal of another panel and the negative terminals of the two panels are connected together.
Connect this cable to the mc4 male adaptor.
When stringing panels in series each panel additional adds to the total voltage v of the string but the current i in the string remains the same.
Connect the negative wires together in the same fashion and the wires off the last panel in the parallel circuit are connected to the battery.
Make sure that the locking tabs snap in for a firm connection.
The bare end of the positive extension wire will now be of positive polarity.
Set the volt meter to read dc volts.
Wire solar panels in parallel by connecting the positive wire of the first solar panel to the positive terminal of the second panel and so on.
Actually there is a color code standard for dc pv systems and as the nec code book does not specify ac or dc the color code applies equally to both such systems.
A simple voltage reading will show you the polarity of a solar panel even when inside.