MPPT solar charge controllers are rated in amps (Output Current). To select a charge controller, you’ll need to calculate the maximum amount of current (in Amps) that the MPPT should be able to output.
This max output current value is calculated by dividing the maximum system wattage (in Watts) by the minimum charging voltage of the battery bank (in Volts).
In other words, we calculate how much current the solar charge controller needs to be able to put out by using this simple formula:
MPPT amperage rating = (Max. System Wattage) / (Min. Battery Charging Voltage)
However, MPPT charge controllers also have a Maximum Input Voltage rating, which indicates the maximum amount of voltage (in Volts) that is acceptable at the input of the MPPT.
So, when selecting your solar charge controller, you should account for both current and voltage.
But, don’t worry! I got you covered…
To make your life easier, I’ve made an MPPT size calculator that will do all the heavy lifting and give you a direct link to the charge controller best suited for your needs.
Below the MPPT calculator, I’ll give you 3 examples of different size solar arrays and battery banks that’ll illustrate this for you.
Also if you want a more in-depth understanding of how the calculator works, please refer to our MPPT sizing guide.
MPPT Size Calculator
The MPPT calculator has 6 input fields that will describe your solar energy system:
1- Solar panel wattage: This is the watts rating on each of your solar panels.
2- Solar panel open-circuit voltage (Voc): You can find this value in the specification label on the back of your solar panels, or by looking up the specific model.
3- Battery bank voltage (Nominal Voltage): The voltage of each battery is usually written on the casing. If you have more than one battery, the voltage of the battery bank is equal to the voltage from one string of batteries (series).
4- Lowest temperature during sunlight hours: In this field, you should enter the lowest value of temperature that you estimate your solar panels are ever going to be exposed to.
5- Number of strings: In your solar array, how many parallel strings are there?
6- Number of solar panels in each string: In each string, how many solar panels are wired in series?
As promised, below you’ll find a few examples that’ll clear things up for you.
- In these examples, I’ll use the 100W-12V Monocrystalline Solar Panels from Renogy. The manufacturer specifies 22.3V as an open-circuit voltage for these panels.
- For the batteries, I will use these 100 Ah 12V LiFePO4 Deep Cycle Battery from Battle Born.
- I will also assume the lowest temperature during sunlight hours these panels in the examples will ever be exposed to is estimated at -3°F.
Example 1: 200W-12V solar array with a 12V battery bank
For the first example, we have 2 100W-12Vwatts solar panels, these panels are wired in series and need to charge a 100Ah-12V Battle Born battery.
Now we need to select the right size MPPT charge controller for this system.
So what do we know so far?
- We have 2 100W-12V solar panels wired in series.
- Our battery bank’s voltage is 12V nominal.
We still need 2 pieces of information:
- The open-circuit voltage (Voc) on these panels.
- The lowest temperature these panels will be exposed to.
As mentioned above, the manufacturer specifies 22.3V as the open-circuit voltage for each of these panels.
The lowest temperature in our example is -3°F.
Now that we have all the information we need, let’s take a look at the results from the MPPT calculator.
The MPPT calculator tells us that our solar charge controller needs to have a maximum voltage input of more than 53V, and needs to be able to put out 22.5 amps.
The calculator also gave us links to 2 choices for MPPT charge controllers that meet these criteria.
Example 2: 400W-24V solar array with a 12V battery bank
For the 2nd example, we have 4 100W-12V solar panels, these panels are wired in 2S2P (2 parallel strings with 2 solar panels in each string).
These panels need to charge 2 parallel wired 100Ah-12V batteries.
So what we know is:
- We have 2 parallel strings.
- 2 solar panels in each string.
- The power rating of our solar panels is 100W.
- The open-circuit voltage of our solar panels is 22.3V.
- The voltage of our battery bank is 12V.
- The lowest temperature is -3°F.
For this system, the MPPT calculator suggests a Victron 100V-50A charge controller and an EPEVER 50 amp charge controller.
Both of these charge controllers can handle the anticipated 53 Volts at their input and can put out up to 50 Amps of current.
Example 3: 200W-24V solar array with a 24V battery bank
For the third example, we have 4 100W-12V solar panels. And same as the 2nd example, these panels are wired in 2S2P.
However, the solar panels in this system need to charge 2 series wired 100Ah-12V batteries.
So for this example:
- We have 2 parallel strings.
- 2 solar panels in each string.
- The power rating of our solar panels is 100W.
- The open-circuit voltage of our solar panels is 22.3V.
- The voltage of our battery bank is 24V.
- The lowest temperature is -3°F.
Related topics:
What size wire between solar panels and MPPT?
What size wire between the MPPT and the battery?
Solar panels in series and in parallel – What is the difference?
