Sizing MPPT Charge Controllers

How to Size a Solar Charge Controllers

Solar charge controllers are rated and sized by the solar panel array current and system voltage. Most common are 12, 24, and 48-volt controllers. Amperage ratings normally run from 1 amp to 60 amps, voltages from 6-60 volts.

For 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. Looking through the products we find a 20-amp controller, as close a match as possible.There is no problem going with a 30-amp or larger controller, other than the additional cost. If you think the system may increase in size, additional amperage capacity at this time should be considered.

MPPT Charge Controllers

Outback Power's MX60, an MPPT Charge Controller

Traditionally, you would assume that the nominal voltage of your battery and your solar panel array would be the same and that you would also choose that voltage for your charge controller. However, in recent years, a more efficient charging technology called Maximum Power Point Tracking (MPPT) has become available on some models of charge controllers. What of interesting features of this technology is that it usually allows you to have a solar panel array with a much higher voltage than your battery bank's voltage. The MPPT charge controller will automatically and efficiently convert the higher voltage down to the lower voltage.

MPPT Charge Controllers Save You Money on Wiring Costs

A big advantage to having a higher voltage solar panel array is that you can use smaller gauge wiring to the charge controller. And since a solar panel array can sometimes be over a 100 feet away from the charge controller, keeping the cost of the wiring down to a minimum is usually an important financial goal for the whole project. When you double the voltage (e.g. from 12 to 24 volts), you will decrease the current going through the wires by half which means you use a quarter as much copper (or cable with half of the diameter). See our wire sizing seminar for more information.