What Happens if Your Solar Charge Controller is Too Small?

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Charge controllers are critical components in solar systems. No matter how powerful your solar panels and batteries are, you won’t get peak performance if the controller is too small. So let us dive deep into charge controller sizing and why it matters.

If the charge controller is too small for the solar panels, the charging and load output will be limited. The charge controller capacity should be greater than the solar panels to eliminate energy and capacity waste.

How Charge Controllers Work and Why Size Matters

Charge controllers regulate the flow of current in a battery. As solar panels convert energy into electricity, the current is stored in the battery. The controller ensures the battery does not get overloaded.

The charge controller amp capacity determines how much power goes into the system for charging and loading. If your solar system has a 50A output capacity but the controller is only 30 amps, only 30 amps goes into the system. The rest is wasted.

There is no risk of overcharging the controller. The controller is going to restrict the output to what it can handle. However it is going to result in a lot of energy loss.

If you have a large solar array and battery, the charge controller must be the right size. You can even have an extra large capacity controller in case you plan to expand the solar array.

If you are going to purchase a solar panel kit, it will probably include a charge controller. The Topsolar Solar Panel Kit for example, includes a 100 watt solar panel and a 20A 12V/2V charge controller. No need to figure out what controller size to use since it is already included.

But if you want to buy each piece separately and build your own solar system, the following information can help.

What Charge Controller Size Do I Need?

The following is a general guideline to find out what solar controller size your PV panels and battery will need. But yours might be different so refer to the instructions that came with your system.

To find what charge controller size you need:

Total solar array watts / battery voltage + 25% = solar charge controller size

If you have a 300 watt solar array and a 24V battery, a 20A charge controller is sufficient.

300 / 24 = 12.5

12.5 + 25% = 16.6

So a 300 watt solar panel or array needs a minimum 16.6A charge controller. The nearest available size is 20A which should be enough.

The 25% in the calculations is to compensate for energy losses, system inefficiencies, temperature, environment etc. You can set this number lower, but 25% is ideal in most cases.

Charge Controller Voltage Explained

Most solar controllers are available in 12V, 24V and 48V. Large capacity controllers are designed for high capacity solar panel and battery systems, so make sure yours matches up properly.

For example, most 60A charge controllers are designed for 24V batteries and solar panels. This isn’t a serious problem though. Connect two 12V solar panels in a series and you can use them with a 24V battery.

To keep things simple, a 12V charge controller should be used with a 12V system, a 24V controller with a 24V system and so on. The controller voltage and amp capacity must be a match for the solar panels and battery. If the controller is not big enough, your system will not function at its optimum level.

Keep in mind that a 12V solar panel can go up to 18V when running, and a 24V panel may reach 36V. 12V and 24V are nominal voltages, but their actual voltage when running is higher.

That is another reason why we add 25% to the controller size calculation. Adding a cushion is standard in solar panel and battery size calculations to prevent overloading.

Again this number crunching is only needed if you are buying the controller separate from the solar panel. If you purchased an all in one kit you should have compatible controller, solar panels and connectors.

How Many Watts Can a Charge Controller Handle?

Charge controller sizes are measured in amps so figuring out the capacity is easy. But what about the watts? How many solar panels for instance, can a 30A charge controller handle?

The formula is very simple:

amps x battery volts = charge controller watt capacity

If you have a 12V 30A charge controller, it can only work with 12V batteries, so:

30 x 12 = 360

Using the same formula we can estimate the watt capacity of various charge controller sizes.

12V Charge Controller Watt Capacity

  • 10A 120W
  • 20A 240W
  • 30A 360W
  • 40A 480W

24V Charge Controller Watt Capacity

  • 40A 960W
  • 60A 1440W
  • 80A 1920W
  • 100A 2400W

Some 40A controllers like the Renogy Rover are compatible with 12V and 24V. But 60A and larger charge controllers only work with 24V and 48V systems, so the formula is charge controller amp size x 24V = watt capacity. If you have a 48V system use the same steps except replace 24 with 48.

Now you have probably seen some controllers that can handle more. For instance there are 40A charge controllers with a 520W limit for 12V and more than 1000W for 24V.

This comes down to design and the system specs. High quality charge controllers may be able to handle more power.

There is also the fact that solar panels do not produce peak output throughout the day. All solar panels no matter the size only produce their rated output for a few hours a day, assuming ideal conditions.

So if a solar panel does not generate maximum capacity, the charge controller does not run at full capacity either. So it might be able to handle a high amount of current for short periods.

This is similar to an inverter that can run a load higher than its stated maximum limit. This could work but only for short periods and if done infrequently.

If you are not sure, check the instructions that came with your charge controller. Even if it has a stated solar array wattage limit, it is best not to push the controller to its capacity. Having extra amps in reserve is ideal in the event of a power surge.

Features to Look For in a Charge Controller

  • MPPT vs. PWM: if you use 200W or more solar panel power, use an MPPT charge controller. They are more effective and will ensure you get the maximum output from the panels and the battery. PWM controllers are limited to 14.4V, and anything higher than that gets discarded. If you have a high powered system, get an MPPT controller.
  • Overcharge protection. Any charge controller worth its salt must have this feature. Whether it is 12V, 24V or 48V, there must be a mechanism that protects the battery from being overloaded by solar energy. This is one of the most important features any controller should have.
  • Low maintenance. A well designed charge controller should not need any maintenance. Once installed the device will run on its own.
  • Display indicator. The information should include details on the current flowing into the battery, temperature, charging capacity and more. The display will also notify you if there are problems with the the controller or battery.
  • Auto charge notification. This is self explanatory. The controller will notify you when the battery is fully charged. There should also be other notifications for other important functions.
  • Reliability. The device should run for several years without any problems. In this case, buying from a reliable manufacturer is the most important thing.
  • Reverse flow monitor. In rare cases, a battery might move the current back out into the panel, a phenomenon known as reverse current flow. This is probably one of the reasons why it would appear that batteries drain overnight . With a built in monitor, you can detect and prevent this from happening.
  • Limit maximum charge. This is another self explanatory feature. The charge controller will limit the charge going into the battery, protecting it from overheating, overcharging and overloading.


Charge controllers are usually bundled in solar panel kits so you don’t have to worry if it is too small for your solar system. If you are going to buy the components separately, the guidelines given here should help you find the right capacity.