Broadly speaking, parallel arrays of nominal 12v panels are used with pwm charge controllers and serial arrays or higher nominal voltage panels used and MPPT controllers.

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⁾
• 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