• With MPPT solar charge controllers and 12v house battery bank there is often little practical difference between parallel and series solar arrays.
• PWM controllers run arrays of the same nominal voltage as their battery bank: 12v and 12v, 24v and 24v.
• generally speaking, parallel panels will perform better in partial shade than series. [but don't shade your panels in the first place – secessus]
• as with other series/parallel arrangements, series increases voltage and parallel increases current.
• series arrays can reduce capital costs in some circumstances
• series/parallel arrangements are described with xPxS nomenclature:
  • 2 panels in parallel would be 2P1S
  • 2 panels in series would be 1P2S
  • 4 panels in series-parallel would be 2P2S

Series arrays are used to generate higher voltages to feed the controller.

• PRO
  • higher PV voltage can run in relatively thinner wires to the charge controller
  • serial connections can provide some charging in low light situations where lower voltage parallel or single-panel connections might not. Imagine two nominal 12v panels running at 11v at sunrise; the series arrangement might yield ~22v (battery charging) instead of 11v (voltage too low to charge battery). Note that power harvestable at those times may be quite low.
  • higher-voltage serial configurations with MPPT controllers may provide more power under partial shading
  • higher voltages work well with MPPT controllers
  • higher voltages can charge higher voltage battery systems like 24v.
  • more efficient than parallel when mixing PV of same Imp but different Vmp¹⁾
  • slightly better than parallel when mixing PV of different Imp and Vmp²⁾
  • higher voltages can help overcome minimum delta requirements to get the MPPT algo running (Victron controllers, for example, require +5v to start and +1v to maintain MPPT).
• CON
  • series arrays can generate quite high operating voltages. Be sure your controller can handle the VoC with edge effect.
  • requires MPPT charger to get full use of high-voltage arrays under normal conditions.
  • thin film panels are generally not recommended for use in serial arrays
  • short serial strings are more susceptible to partial shading losses than parallel

• PRO
  • less impacted by partial shade³⁾ when using PWM controllers
  • more efficient than series when mixing PV of same Vmp but different Imp⁴⁾
  • can use charge controllers with lower max voltage input rating
  • can use simpler, less expensive PWM charge controllers with nominal 12v panels without losing much efficiency
• CON
  • may not provide sufficient voltage to charge battery during very bad conditions (insolation < 20%)
  • may not provide sufficient voltage to charge higher voltage (20 or 24v) battery banks

Consider this scenario:

• 4 panels
  • 150w
  • Vmp == 19v
  • Voc == 22.8
  • Imp == 7.9A
• MPPT controller with 60v maximum input voltage

A fully-series configuration (1p4s) would have a Voc of 91.2v, greatly exceeding the controller's input spec of 60v, and it's common to want to have 20% voltage headroom above Voc.

A fully-parallel configuration (4p1s) would require 3x 2-into-1 MC4 adapters or 1x 4-into-1 adapter. The wiring from the array would need to carry 30A+ to the controller, necessitating heavier wiring that would cost more and might not even fit the controller terminals. It might be unable to charge at all under extreme low-light conditions.

A compromise might be series-parallel array (2p2s) with 45.6Voc and ~16A. This would require 1x 2-into-1 MC4 adapter to combine the two serial strings. Voltage is low enough for the controller, but high enough to allow low-light charging and the use of lighter wiring.

• Solar Panels – Series vs Parallel by Explorist Life
• How To Wire Solar Panels In Series vs Parallel -- For Beginners
• testing series vs parallel in a marine environment