MPPT Charge Controllers allow us to stack panel voltages by connecting them in series. MPPT chargers are rated with two numbers, for example a 75/15 charger allows a maximum of 75 volts and a maximum 15 amps. We can stack as many panels in series until 75 volts, keeping the amperage as low as possible. In reality a 12v panel may produce up to 22v so it best to measure the array with a multimeter before connecting to charge controller. Panel voltage must be 5 volts higher that battery voltage.

Reminder that with a 12v battery a 75/15 charge controller will only be able to handle 180 watts worth of solar (15 amps X 12 Volt). With a 24 Volt battery the same charge controller can now handle 360 watts worth of solar.