The short version of MPPT is that panels
typically put our about 17.7 volts at their rated amperage in full sunlight.
Since batteries will take a charge at somewhere around 14.4V, a normal regulator
will throttle back the 17.5 volts to the batteries from the solar panels
converting it to heat instead and wasting avilable power.
MPPT controllers instead convert the excess voltage to amperage and give you more amphours per day into your battery
as a result. Overall gains are typically in the 10-15% range on average.