**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-sized 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.

###### I get commissions for purchases made through links in this post.

## 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?

## MPPT Solar Charge Controller Calculator

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?**

i recently bought a 200 amp, 12volt batter with blue tooth, 40 amp Renogy charge controller, 2-100 watt solar panels. from your examples above with 4-100 watt panels, i could add 4 more panels to my system without replacing my charge controller for a 60 amp or higher.

Am i understanding thjis correctly? thanks Steve

Hey there Steve,

Before you make any decisions, I suggest testing the number of solar panels that you intend to add in the calculator.

Thanks Younes you have done a very good job in educating readers about MPPT. However, you have stopped short of explaining the MPPT calculator which I believe will impact on practicing engineers like me. I wish more effort.

Abba

Very comprehensive, thank you! As it turns out, I am already using the Renogy 200W Kit (2 panels w/20A Rover), and now have 6 lead-acid 6V batts in 3banks. I’m having a problem keeping the charge up (52-70%), and thinking that I am not getting enough solar to replenish the batteries over a 4-5day stay. I’m not sure how to look at this, as I am trying to decide if I have too many batteries (to decrease the batts), or not enough solar panels (add panels)!?

Hey there Mike,

From what I understand, you have 6 Lead-Acid batteries each rated at 6V-100Ah, and each 2 of these are wired in series to make a 12V-100Ah battery. This means that you have a 12V-300Ah battery bank in total, making your total rated Energy Capacity 3600 Watt-hours (3.6 kWh).

On the other hand, you only have 200W of solar, which means that at most you’ll be getting 1200 Wh of energy on a daily basis. This is, of course, assuming optimal sunlight.

This means that even if you don’t discharge your batteries at all, you’ll need 3 days to take the battery from 0% to 100%.

So, to answer your question, you need more solar panels.

Hope this helped.

Hi I have 4 200w panels 800w Open Circuit Voltage (Voc): 21.6V is my Victron mppt 150 70 tr over the top and probably would not work 100%

Hey there Brian,

Have you specified the Nominal Voltage of the battery and the configuration of the solar panels (series/parallel/combination of both) to the calculator?

If you do, it should provide a couple of links to appropriate MPPTs.

Thank you very much for prviding this information. have a great day

Hi,

I want to know about the lowest temperature during daytime. Is it the lowest temperature for where I live on yearly basis?

By the way Thanks for your articles. I was looking for these informations.

Hello Tom,

Yes, I’m referring to the lowest temperature possible when the solar panels are capable of producing voltage, which is only possible in daytime hours. If you don’t have an exact data point, just put an estimate.

Glad to have helped.