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Sizing Common Solar Charge Controllers

All renewable energy (RE) systems with batteries should include a charge controller. In this article we’ll principally be referring to solar charge controllers . Charge controllers prevent battery overcharging and also prevent the batteries from sending their charge back through the system to the charging source (i.e., the solar panels). Think of a solar charge controller as a battery nurse—its job is to monitor the battery bank, feeding it what it needs and checking its vital signs. Since a solar controller does its work in line between the solar panel array and the batteries, it would make sense that its selection and sizing would be influenced by those components. And that’s exactly the case.

Voltage and amperage (or current) are the parameters we use in solar charge controller sizing. The solar  controller must be capable of accepting the voltage and current produced by the DC source (usually solar panels) and delivering the proper voltage and current to the batteries. This situation might make you think that the DC source, charge controller and batteries must all share a common voltage. While that is one system design strategy used in many installations, it’s not the only one. More on the alternatives in another article. For now, it’s one voltage for everyone!

Technically speaking, the DC source must always have a higher operating voltage than the battery bank in order for current to flow from one to the other. A handy way to remember this fact is the statement, “curent flows downhill.” For the purpose of this discussion, we’ll use nominal voltage which means common battery voltages. Nominal voltage in this sense is synonymous with battery voltage. Since batteries (where they are used) are in many ways the heart of an RE system, we can call the bank’s voltage the system voltage. The system voltage selected for any given installation is usually, though not always, determined by the battery bank required by the application; the inverter, if one is used, will also influence the choice of system voltage.

Sizing comes down to this: there’s the quick method, which will very likely give an acceptable, if perhaps oversized result, but won’t describe the why here we’ll just show you the steps.

Short Method for Sizing Solar Charge Controllers

  1. For PWM and PWM shunt solar controllers, select one that is rated at your system voltage (same nominal voltage all the way through the system).
  2. Divide solar panel array total wattage by system voltage.
  3. Add 20% as a safety margin (i.e., (result of Step 2) X 1.2).
  4. Select a solar controller rated at or above the result of Step 3.

Example of the Short Method

  1. Two 125W, 12V nominal modules. System is 250 W, 12 Volt nominal. Solar charge controller will be 12 Volts.
  2. 250 ÷ 12 = 20.83
  3. 20.83 * 1.2 = 24.996 amps
  4. You could use a Xantrex C35, a 35 amp solar charge controller; or a Morningstar Prostar 30, a 30 amp solar charge controller , for example. Any 12 volt solar controller greater than 25 amps will work.

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