MPPT charge controllers employ DC to DC conversion, which allows the solar array voltage to be higher that the voltage required to charge the batteries. Many years ago solar modules were designed to charge 12 volt batteries. These solar modules were built with 36 cells in series to have a peak power voltage of 16- 18 volts. Two of these modules could be wired in series to charge a 24 volt battery and four could be wired in series for a 48 volt battery. Modules with 72 cells were also available for 24 volt charging. When all modules were made this way, the simple PWM charge controllers described previously worked fine. As connected off grid solar power systems became the prominent use for solar modules, cost became the most important factor in module design and off the grid homes.
This lead to larger 60 cell modules, manufactured in a convenient size that could be carried by one person. These modules have a voltage too high for 12 volt battery charging and too low for 24 volt battery charging and are usually wired in series to get approximately 400 volts DC which is easy to convert to 240 volts AC for connection to the utility grid.
36 cell modules are available with power outputs up to 140 watts and 72 cell modules are available with power outputs up to 300 watts. These modules cost more per watt, but can be used with lower cost PWM charge controllers. It makes economic sense to use these modules on systems that required less than 700 watts. If the power required is greater than 700 watts, the cost savings on the solar modules and the wire between the modules and the batteries to pay the added cost of a MPPT charge controller.
MPPT charge controllers from Schneider (formerly Xantrex), Outback, Morningstar and Midnite Solar can operate with a maximum input voltage from a solar array of 150 volts, which can be used with three 60 cell modules in series. Other models from Midnite Solar can be used with arrays up to 250 volts allowing the use of six 60 cell modules in series. An added advantage of using a higher voltage solar array made from modules wired in series is that much smaller wire can be used between the solar array and the charge controller without significant power loss. Every time you double the voltage in a wire, you can carry 4 times as much wattage with the same loss. This saving increases tremendously when you go from 15 volts in a wire to 150 volts.
When choosing an MPPT charge controller, the amp rating of the controller is the maximum amperage that it can supply to the battery being charged. To find this amperage, add the wattage of all modules in the array and divide by the battery voltage. For example, six 240 watt modules have a total wattage of 1440 watts. If you are charging a 12 volt battery, the charge controller amps required is 120 amps (1440 / 12 = 120). In this case you would need controllers with 60 amp or greater rating. If you were charging a 24 volt battery, only one 60 amp charge controller would be required. If you are using a 48 volt battery you could have an array of 2800 watts and still use a single 60 amp charge controller.