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.

Now that you know what size controller to look for, identify which type of charge controller is right for your application: MPPT, PWM, and PWM shunt controllers. A PWM (Pulse Width Modulated) charge controller is the traditional style. They are robust, inexpensive and widely used in PV applications. PWM shunt controllers are used less often and mostly in applications where electrical interference is an issue. The MPPT (Maximum Power Point Tracking) charge controller is the shining star of today's PV systems. These controllers actually detect the optimum operating voltage and amperage of the solar array and match that with the battery bank. The result is additional 15-30% more power out of your array versus a PWM controller. Although the MPPT controller is more expensive than its PWM counterpart, it is generally worth the investment for any solar electric system over 200 watts.

Maximum Power Point Tracking - this is electronic tracking, and has nothing to do with moving the panels. Instead, the controller looks at the output of the panels, and compares it to the battery voltage. It then figures out what is the best power that the panel can put out to charge the battery. It takes this and converts it to best voltage to get maximum AMPS into the battery. (Remember, it is Amps into the battery that counts). Most modern MPPT's are around 92-97% efficient in the conversion. You typically get a 20 to 45% power gain in winter and 10-15% in summer. Actual gain can vary widely depending weather, temperature, battery state of charge, and other factors.

Here is where the optimization, or maximum power point tracking comes in. Assume your battery is low, at around 11.5 volts. A MPPT takes that 16.9 volts at 7.1 amps and converts it, so that what the battery gets is no longer 7.1 amps at 16.9 volts, but 9.6 amps at about 12.5 volts. Now you still have almost 120 watts, and everyone is happy.

What an MPPT charge controller does is "look" for that exact point, then does the voltage/current conversion to change it to exactly what the battery needs. In real life, that peak moves around continuously with changes in light conditions and weather.

A MPPT charge controller tracks the maximum power point, which is going to be different from the STC (Standard Test Conditions) rating under almost all situations. Under very cold conditions a 120 watt panel is actually capable of putting over 130+ watts because the power output goes up as panel temperature goes down - but if you don't have some way of tracking that power point, you are going to lose it. On the other hand under very hot conditions, the power drops - you lose power as the temperature goes up. That is why you get less gain in summer.

Winter, and/or cloudy or hazy days - when the extra power is needed the most.

• Cold weather - solar panels work better at cold temperatures, but without a MPPT you are losing most of that. Cold weather is most likely in winter - the time when sun hours are low and you need the power to recharge batteries the most.
• Low battery charge - the lower the state of charge in your battery, the more current a MPPT puts into them - another time when the extra power is needed the most. You can have both of these conditions at the same time.