A charge controller, also known as a solar regulator, is a regulator that goes between the solar panels and the batteries. A charge controller is used to regulate the voltage and current coming from the solar panels and going to the battery. As the input voltage from the solar array rises, the charge controller regulates the charge to the batteries preventing any over charging.

Sizing a Controller:
Charge controllers are rated and sized by the array current and system voltage. Most common are 12, 24, and 48 volt controllers. Amperage ratings run from 1 amp to 60 amps, voltages from 6-60 volts.

To size your controller, here is an example:
If one module in your 12-volt system produces 7.45 amps and two modules are utilized, your system will produce 14.9 amps of current at 12 volts. Because of light reflection and the edge of cloud effect, sporadically increased current levels are not uncommon. For this reason we increase the controller amperage by a minimum of 25% bringing our minimum controller amperage to 18.6. A 20-amp controller is as close a match as possible, however there is no problem going with a 30-amp or larger controller, other than the additional cost. When selecting a solar regulator, it's important to look ahead - if you think you'll be adding to your system, it pays to buy a regulator rated as high as possible, which can save you money when you add additional solar panels.
NOTE: the solar regulator should be capable of handling the total short circuit current of a solar panel.

Generally, there is no need for a charge controller with the small maintenance, or trickle charge panels, such as the 1 to 5 watt panels. A rough rule is that if the panel puts out about 2 watts or less for each 50 battery amp-hours, then you don't need one.

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