MPPT solar charge controller with 12v panels...

Another factor favorable to serial is greater shadow tolerance than parallel. Virtually all panels have bypass diodes that isolate the panel into sections, 3 for my panels. This means that a shadow on a section causes the diode to conduct bypassing that section. In other words the voltage of my panel would be reduced by 1/3 but with full amps. The same effect for parallel would effectively eliminate that panel due to it's voltage decrease.