Use our solar charge controller calculator to easily pick the right size PWM or MPPT charge controller for your DIY off-grid solar panel system.
Solar Charge Controller Calculator
- This calculator assumes all your solar panels are identical.
- If you don’t enter a Temperature Coefficient of Voc, the calculator assumes that your panels are monocrystalline or polycrystalline silicon solar panels, which are the predominant types of solar panels on the market today.
- The given MPPT charge current rating assumes the controller can be over-paneled.
What Size Charge Controller Do I Need?
Sizing your charge controller is one of the hardest parts of designing a DIY solar system.
But, after years of learning and refining how I design my projects, I’ve simplified the entire process down to 4 main steps:
- Calculate Solar Array Wattage
- Calculate Max PV Voltage
- Calculate Max Charging Current
- Check for Compatibility
Let’s run through them.
Note: This is a simplified version of my process. For the full breakdown, check out my tutorial on how to size a solar charge controller.
Step 1: Calculate Solar Array Wattage
1. Find your solar panel’s wattage. If you don’t already know its wattage, you can find it on a label on the back of the panel or in its datasheet. It will be listed as Max Power or something similar and abbreviated as Pmax. Here’s the label on one of my solar panels as an example:
2. Multiply your panel’s wattage by the number of panels in your array to get your solar array’s wattage. Let’s say you’re using 4 solar panels:
Solar array wattage = Solar panel wattage × Number of panels Solar array wattage = 100W × 4 panels Solar array wattage = 400W
It’s that easy.
Note: This calculation assumes all your solar panels are identical. If you’re using different solar panels, I recommend using our solar panel series and parallel calculator to calculate your array’s wattage.
Step 2: Calculate Max PV Voltage
A solar panel’s voltage increases as temperature decreases. So, when finding the maximum voltage of our solar array (aka “maximum PV voltage” or “maximum PV open circuit voltage”), we need to account for temperature.
Note: Our solar charge controller calculator at the top of this page does these calculations for you under the hood. You can also use our solar panel maximum voltage calculator, which I’d recommend if your solar panels are not all identical.
1. Find your solar panel’s open circuit voltage (Voc). You can find this number on a label on the back of the solar panel or in its datasheet.
2. Multiply the panel’s Voc by the number of panels you have wired in each series string to find the open-circuit voltage of your solar array. Let’s say I have 2 panels in each of my series strings.
Solar array Voc = Solar panel Voc × Number of panels in each series string Solar array Voc = 22.3V × 2 panels in series Solar array Voc = 44.6V
3. Find your lowest expected temperature. This should be the lowest temperature you expect your solar array to experience in daylight. Often, people will use the lowest recorded temperature at their location. If you can’t find yours, you can use a conservative value of -40°F (-40°C).
Tip: If your solar panels are mounted on a vehicle, consider the various locations you plan on visiting in your vehicle when picking your lowest expected temperature.
1. Find the right correction factor from the table below using your lowest expected temperature. For this example, let’s say your lowest expected temperature is -10°F (-23°C). In that case your correction factor would be 1.2.
Here’s the table:
|Factor||Ambient Temperature (°F)||Ambient Temperature (°C)|
|1.02||76 to 68||24 to 20|
|1.04||67 to 59||19 to 15|
|1.06||58 to 50||14 to 10|
|1.08||49 to 41||9 to 5|
|1.10||40 to 32||4 to 0|
|1.12||31 to 23||-1 to -5|
|1.14||22 to 14||-6 to -10|
|1.16||13 to 5||-11 to -15|
|1.18||4 to -4||-16 to -20|
|1.20||-5 to -13||-21 to -25|
|1.21||-14 to -22||-26 to -30|
|1.23||-23 to -31||-31 to -35|
|1.25||-32 to -40||-36 to -40|
Note: The above table has been adapted from Table 690.7(A) from the 2023 edition of the NEC. It applies to monocrystalline and polycrystalline silicon panels. If you aren’t using mono or poly panels, you must calculate your solar array’s max Voc using temperature coefficient of Voc, which you can do using our calculator at the top of this page.
2. Multiply your solar array’s Voc by your voltage correction factor to get your solar array’s max Voc. I’ll be using the solar array Voc I calculated above (44.6V) and a voltage correction factor of 1.2.
Solar array max Voc = solar array Voc × Correction factor Solar array max Voc = 44.6V × 1.2 Solar array max Voc = 53.52V
Step 3: Calculate Max Charging Current
Calculating max charging current depends on whether you’re using a MPPT or PWM charge controller.
So I’ll break this step down by charge controller type:
MPPT Charge Controllers
Most MPPTs can limit the current coming from the solar array so as to not exceed their charge current rating.
What this means is that we can use a simple calculation to estimate our MPPT max charging current and not worry much about scenarios when our solar array’s current may exceed that number.
Note: Double check that the MPPT you’re considering supports ‘over-paneling’ before using this method.
1. Pick a charging voltage based on your battery’s nominal voltage. A 12V battery doesn’t charge at exactly 12 volts. The same goes for a 24V battery. So, using the table below, pick a charging voltage based on your battery bank’s nominal voltage.
|Nominal Battery Voltage||Common Charging Voltage|
2. Divide your solar array’s wattage by the charging voltage. Watts divided by volts gives us amps. Let’s say I have a 400W solar array and a 12V battery bank.
MPPT max. charging current = Solar array wattage ÷ Charging voltage MPPT max. charging current = 400W ÷ 14.4V MPPT max. charging current = 27.78A
And that’s it!
PWM Charge Controllers
Note: PWM charge controllers should only be used if the solar array and battery bank nominal voltages are identical.
Unlike MPPTs, PWMs can’t limit the current coming from the solar array.
So, to calculate a PWM’s max charging current, we need to find the max current of our solar array. Fortunately, the National Electrical Code (NEC) clearly spells out how to do that.
1. Find your solar panel’s short circuit current (Isc). You can find this number on a label on the back of the solar panel or in its datasheet. In this example, my 100W panel’s Isc is 5.86A.
2. Multiply the panel’s Isc by the number of panels or series strings you have wired in parallel to get the short circuit current of your solar array. Let’s say my solar array has 2 series strings wired in parallel.
Solar array Isc = Solar panel Isc × Number of panels or series strings in parallel Solar array Isc = 5.86A × 2 series strings in parallel Solar array Isc = 11.72A
3. Multiply your solar array’s Isc by a safety factor of 1.25. This safety factor comes directly from the NEC and is meant to account for times when solar panels output more than their rated power. (This can happen occasionally under conditions such as the cloud-edge effect.)
PWM max. charging current = Solar array Isc × 1.25 PWM max. charging current = 11.72A × 1.25 PWM max. charging current = 14.65A
Step 4: Check for Compatibility
2. Check that the charge controller is compatible with your battery bank. It should be compatible with your battery voltage. It should work with your battery type (e.g. LiFePO4, sealed lead acid, flooded lead acid) or support custom charging profiles. And, if it’s a PWM, your battery nominal voltage and solar array nominal voltage should be identical.
3. Check that the charge controller is compatible with your solar array. Its maximum PV input power should be greater than or equal to your solar array wattage. Its maximum PV input voltage should be greater than or equal to your solar array’s maximum Voc. And its charge current rating should be greater than or equal to your maximum charging current.
If it passes these compatibility checks, then you know the charge controller is properly sized for your solar system.
But, before you buy, I’d recommend considering the following:
Other Things to Consider When Picking Your Charge Controller
- Extra features: Consider usability features such as Bluetooth monitoring, load terminals, and whether or not the controller has a screen.
- Battery temperature range: If your batteries will get cold, look for temperature protection features. LiFePO4 batteries that experience below-freezing temperatures should either have a feature called low-temp cutoff, or the charge controller should be able to stop charging below freezing. Lead acid batteries that experience wide temperature ranges should be paired with a charge controller that has a feature called temperature compensation to maximize battery lifespan.
- Charge controller specs: Take a look at specs such as max wire size if you like to over-gauge your wires, and IP rating and operating temperature range if your charge controller will experience extreme temperatures or marine environments.