In general, parallel arrays of nominal 12v panels are used with PWM charge controllers and serial arrays with higher nominal voltage panels 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 partial shade situations where lower voltage parallel or single-panel connections might not. Imagine two nominal 12v panels running at 11v in heavy cover:¹⁾ the series arrangement might yield ~22v (battery charging) instead of 11v (voltage too low to charge battery). This also happens in low, even light (like dusk or dawn) but the amount of power harvestable at those times is so low as to be of little practical use.
  • 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 Vmp²⁾
  • slightly better than parallel when mixing PV of different Imp and Vmp³⁾
• 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
  • more susceptible to partial shading problems than parallel

• PRO
  • less impacted by partial shade⁴⁾ when using PWM controllers
  • more efficient than series when mixing PV of same Vmp but different Imp⁵⁾
  • 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