Basically, to choose a charge controller correctly, you need to know a little bit of ohms law in physics. This law states that the current flowing through a conductor is directly proportional to the voltages between the two endpoints, with resistance as the constant of the proportionality.
Step 1; Take note of the current
Current is measured in amperes, abbreaviated as amps.
Step 2: Voltage
mostly manufacturers indicate the number of volts generated by their devices. Hence, you might not have to do the measurements manually. Besides, there are several online calculators which got your back.
Step 3: Calculate the power
With current flowing through the circuit and voltage, you can quickly get the power generated by multiplying the two. Power is given by voltage in volts multiplied by the current in amps and labeled in watts.
The most important thing to note is that you should never operate a charge controller to its maximum voltage. You should not use a 10 amp charge controller on a 10 amp solar panel to avoid overloading it and increase its efficiency. Instead, the size of the charge controller should be slightly larger than the solar panel.
Additionally, charge controllers have varying voltages. The most commonly used values are 12v, 24v, and 30 volts. Therefore, you should be careful about that when doing calculations.
The below table shows the watts various charge controllers can handle.
| Charge controller voltage | charge controller current | Maximum Watts, the charge controller, can handle |
| 12 volts | 10 amps | 120 watts |
| 20 volts | 20 amps | 400 watts |
| 12 volts | 30 amps | 360 watts |
| 24 volts | 40 amps | 960 watts |
| 20 volts | 50 amps | 1000 watts |
| 24 volts | 60 amps | 1440 watts |
| 20 volts | 70 amps | 1400 watts |
| 20 volts | 80 amps | 1600 watts |
| 24 volts | 90 amps | 2160 watts |
| 24 volts | 100 amps | 2400 watts |
Let’s have a few examples to make you understand better.
How many watts can a 10 amp charge controller handle?
Most of the 10 amp charge controllers have a nominal voltage of 12 volts. However, you might find some devices with a higher voltage. Don’t worry. We will show you how to find the wattage of any 10 amp charge controller.
To begin, you must first check the volt rating of the device. You can find this on the device or in its manual.
Let’s assume that your device has 12 volts. Thus, its power in watts is given by;
12 volts X 10 amps = 120 watts.
Thus, the amount of power that a 12volt 10 amp charge controller can handle is less than 120 watts. In other words, the total power generated by the solar panel should be less than 120 watts.
Similarly, if you have a 24 volt 10 amp charge controller, its power is given by;
24 volts X 10 amps = 240 watts.
Consequently, the amount of watts that a 24 volt 10 amp charge controller can handle is less than 240 volts to avoid overloading it.
How many watts can a 20 amp charge controller handle?
The nominal voltage of the charge controller should be equal to the solar panel’s voltage. Generally, the amount of voltage generated by the charge controller limits the number of watts that a controller can handle.
Most of the 20 amps charge controllers are designed to run 12 volts and 24 volts systems. Let’s calculate the maximum amount of watts it can run.
Power in watts is given by multiplying the voltage and current.
That is;
12 volts X 20 amps = 240 watts
24 volts x 20 amps = 480 watts
Therefore a 12 volts 20 amps charge controller can handle up to 240 watts while a 24 volts 20 amp charge controller can handle up to 480 watts.
How many watts can a 30 amp charge controller handle?
Indeed, I can easily state how many watts a typical 30 amp charge controller can handle. But what will happen if your voltage ratings are not the same?
Two things might happen; one, you might pick a small device that risks getting all your system burnt or spend several other minutes searching online.
Let’s do the calculations to help you understand the origin of the said values.
Let’s assume that we have 12-volt and 24-volt charge controllers. You can look at the manual or online to determine the voltage generated by your device.
Thus, the power generated is-
12 volt X 30 amps = 360 watts
24 volts X 30 amps = 720 watts
Hence, the amount of watts that a 12 volt 30 amp charge controller can handle is 360 watts, while a 24 volts 30 amps device can handle a maximum of 720 watts.
However, its ideal solar panel power should be less than 720 watts.
How many watts can a 40 amp charge controller handle?
A 12 volt 40 amp charge controller generates 480 watts. That is;
12 volt x 40 amp = 480 watts.
Similarly, a 24 volt 40 amp charge controller has a power rating of 960 watts. That is;
12 volt x 40 amps = 960 watts.
However, the surrounding environmental conditions highly affect the charge controller operation. Considering that and avoiding overloading the system, a charge controller’s watts should be slightly less than its maximum wattage.
Consequently, a 12-volt 40 amp can handle less than 480 watts, while a 24-volt 40 amp charge controller can handle less than 960 watts.
How many watts can a 50 amp charge controller handle?
A charge controller protects the battery from short circuits, power loss at night, and high-voltage surges. However, it should not be overloaded for this device to function efficiently. In other words, its power should be equivalent to the solar panel’s power.
However, considering the effects of the environmental conditions, the power of the charge controller should be slightly higher than the solar panel wattage.
For instance, a 12-volt 50 amp can handle 600 watts, while a 24-volt 50 amp can handle 1200 watts.
However, their ideal solar panel watts should be slightly less than 600 watts and 1200 watts for a 12 volt and 24 volt 50 amp charge controller.
How many watts can a 60 amp charge controller handle?
If you have a 20 volt or 24-volt charge controller, the amount of watts it can handle is given by;
20 volts x 60 amps = 1200 watts
24 volts x 60 amps = 1440 watts
Therefore a 20 volt and 24 volts 60 amp charge controller can handle 1200 watts and 1440 watts, respectively.
How many watts can a 70 amp charge controller handle?
Similarly, a 20-volt 70 amps can handle 1400 watts, while a 20-volt 70 amps can handle 1680 watts.
Below are the calculations;
20 volts x 70 amps = 1400 watts
24 volts x 70 amps = 1680 watts
How many watts can an 80 amp charge controller handle?
To get the number of watts that an 80 amp charge controller can handle, you should multiply the voltage rating with the current generated.
For instance, a charge controller with a nominal voltage of 20 volts or 24 volts can handle;
20 volts x 80 amps = 1600 watts
24 volts x 80 amps = 1920 watts
How many watts can a 90 amp charge controller handle?
The watts that a 90 amp charge controller can handle are given by multiplying the nominal voltage by the current.
For example, a 20 volts 90 amp charge controller can handle 1800 watts.
That is;
20 volts x 90 amps = 1800 watts.
Similarly, a 24 volt 90 amp charge controller can handle 2160 watts.
That is;
24 volts x 90 amps = 2160 watts.
How many watts can a 100 amp charge controller handle?
Lastly, if we have a 20 volt and 24 volt 100 amp charge controllers, the amount of watts that they can handle is given by;
20 volts x 100 amps = 2000 watts
24 volts x 100 amps= 2400 watts.
In other words, a 20 volts 100 amps charge controller can handle 2000 watts while a 24 volts 100 amps charge controller can handle 2400 watts.
Eng. Matthew Joseph Nandirio is the Founder of walkingsolar.
After graduating from the University of Houston in 2002, matt started working as a Solar Electrical Engineer for several multi-national solar energy companies.
He has a wide range of experiences including solar system requirement analysis, planning, maintaining, debugging and even solar device development through research.
He now shares his 20 years of expertise through his articles on the walkingsolar website.
Further, he is also the author of two books on Solar Technology, “Solar Power for Villages” and “DIY Solar System for Dummies”.
