Example of Sizing an MPPT Charge Controller
So, for instance, you could have a 1000 watt solar panel array that operates at 48 volts DC and your battery bank is 24 volts DC. MPPT charge controller are rated by the output amperage that they can handle, not the input current from the solar panel array. To determine the output current that the charge controller will have to handle we use the very basic formula for power (watts), which is: Power = Volts x Amps Here we know the power is 1000 watts, the battery bank is 24 volts, so: 1000 watts = 24 volts x Amps which gives us: Amps = 1000 watts/ 24 volts Amps = 41.7A We still want to boost this value by 25% to take into account special conditions that could occur causing the solar panel array to produce more power than it is normally rated for (e.g. due to sunlight's reflection off of snow, water, extraordinarily bright conditions, etc). So, 41.7A increased by 25% is 52.1A. In this case we'd probably choose a 60 Amp MPPT Charge Controller, like Outback Power's FM60 or Xantrex XW MPPT60 Be aware that MPPT charge controllers have an upper voltage limit that they can handle from the solar panel array. It's important that you make sure than there is no condition that the solar panel array voltage will go above this limit or you will like burn out the controller. You want to make sure that the open circuit voltage of the solar panel array does not go above this limit. You should also give yourself a little bit of a margin for an error to take in account the possibility that a solar panel array's voltage will actually increase the colder it gets. The voltage multiplier is based on the lowest expected ambient temperature where the system is to be installed. The NEC gives Table 690.7 for ambient temperatures below 25 degrees celcius. Most of the higher end charge controllers from Outback and Xantrex have an maximum Open Circuit Voltage (VOC) of 150VDC, the exception is the Apollo T80HV which has a VOC of 200 VDC. You want to make sure that the open circuit voltage of the solar panel array does not go above this limit. Here's an example: We'll use four 12 volt Evergreen 102 Watt solar panels all run in series for a nominal voltage of 48 volts and our battery bank is at 12 volts. We'd like to use BZ Product's MPPT500 charge controller because of its price. If we look at the panel's specification page we see that each panel has an open circuit voltage of 21.3V. That means the array has four times that (because there are 4 panels in series). So the array open circuit voltage is 21.3V x 4 = 85.2V. In central Oregon our lowest min recorder temperature is -26 dgreeF so we would multiply the 85.2V by our multiplier which is 1.25 and we get 106.5V. Now we'll look at the MPPT500's specifications and we see that it can only take a maximum of 100 volts. So we can't use that product in this instance and would need to use a product with a higher VOC such as the Outback FM60 or Xantrex XW MPPT60. However we could arrange the panels in two series string of two panels which would reduce our VOC to 53.25V