PWM charge controller sizing and selection

A PWM charge controller is rated to operate at a particular system voltage and maximum current. PV modules designed to work at the system voltage must be connected in parallel in order to achieve the minimum PV source size and the charge controller therefore must be sized to handle this amount of current. If the current rating of a PWM charge controller is exceeded, it can be damaged or destroyed.

Step 1: Determine PV module power rating

The chosen system voltage limits the choices of modules and configurations that are possible:

• 12 volt system = 1 × 36-cell module per string.
• 24 volt system = 1 × 72-cell module per string or 2 x 36-cell modules in series per string.
• 48 volt system = 2 × 72-cell modules in series per string or 4 x 36-cell modules in series per string.

36-cell modules are typically below 150W. 72-cell modules are typically 300W-400W.

Step 2: Calculate required number of PV modules

This calculation will give a minimum number of PV modules. Different modules sizes and configurations can be explored to find the optimal design.

Required number of PV modules	= minimum PV source size ÷ PV module power rating (Step 1)

The final PV source size should always be larger than this value, thus if the the result of the calculation is a decimal, it should be rounded up.

Step 3: Determine minimum current rating of charge controller

This calculation will give a minimum current rating to use as a basis for selecting the charge controller. The ISC rating of the PV module can be found on its specifications sheet.

Minimum number of PV modules	= minimum number of PV modules (from Step 2) × ISC rating of chosen module (from Step 2) × irradiance safety parameter

Step 4: Select a charge controller

The final chosen charge controller should function at the system voltage and have a current rating that is larger than the minimum current rating calculated in step 3.

Step 5: PV source power rating

The total power rating of the PV source can be calculated by multiplying the power rating of the chosen PV module by the final number of PV modules (Step 2)

PV source power rating	= PV module power rating (Step 1) × minimum number of PV modules (Step 2)