A charge controller is required to store energy from a 300 watt solar panel in the battery bank.

Knowing how to compute the size is crucial because if the controller and panel are not correctly matched, it will not operate.

Fortunately, the procedures are rather simple.

A 30A charge controller is necessary for a 12V 300 watt solar panel, assuming the controller is compatible with the voltage of the system’s batteries.

The majority of 30A charge controllers are designed to function with 12V and 24V batteries, however a bigger one is needed for 48V batteries.

How to Determine the Size of a Charge Controller

Amps are used to describe charge controllers.

The fundamental criterion is that the solar panels’ or solar array’s amps must be more than the controller amp rating.

The equation is:

Solar panel watts / volts = amps + 20% = charge controller size

Thus, the equation for a 12V 300 watt solar panel is as follows:

300 watts / 12 volts = 25 amps plus 20% = 30

For this system, a 30 amp charge controller is required.

Renogy 12V/24V 30A MPPT Solar Controller is our selection.

This controller is compatible with AGM, gel, and lithium batteries as well as 12 and 24V systems.

Some professionals advise using 25% as opposed to 20%.

A 25 percent choice would provide 31 amps, or a 35A controller.

You have an option, but often a 20 percent safety buffer suffices.

This formula takes 12 volts as the system’s operating voltage.

Although the outcomes will be different if you use a 24V system, you may still follow these instructions.

For instance, you would want a 60A charge controller if you connected four 300W 24V solar panels in series.

4 x 300 = 1200

1200 / 24 = 50

50 divided by 20% yields 60.

In Solar Panels, What Do VMP And LMP Mean?

The VMP (voltage maximum power) and the LMP are two figures you should look up in the solar panel specs (maximum current).

For 300 watt solar panels designed for 12V, the VMP is typically 18V, and the maximum current is 5.7A.

Technically speaking, a 12V solar panel operates higher than 12V, but batteries also charge higher than their voltage in this situation.

Check your panel specifications first since higher rated systems could have a 37–40 VMP and 8A maximum current.

You’ll learn why these figures affect how much electricity a solar system can produce in the section that follows.

Which Kind of Charge Controller Should I Use?

A PWM charge controller and an MPPT charge controller have distinct advantages and disadvantages, but the way in which they manage electricity from solar panels is crucial in this situation.

If the battery voltage is comparable, a PWM charge controller is the best option for a 300 watt 12V or 24V solar panel.

Use an MPPT charge controller if the solar panel voltage is much greater than the battery voltage.

For instance, a solar panel with a 5.2 LMP is operating at 18V VMP.

The system is powered by a 12V battery that is being charged at 13V.

(the voltage can range from 10.8 to 14.4V).

The system consumes 67.6 watts with a PWM charge controller (5.2A x 13 volts = 67.6).

As long as the solar panel voltage maintains at 13 volts, the PWM controller will draw this amount of electricity.

In essence, a PWM controller will reduce the solar panel power from 18 V to 13 V and match it to the battery.

The system will draw the full 18V from the panel if an MPPT charge controller is used.

This indicates that 5.2A x 18V Equals 90 watts.

This is a 25% improvement over a PWM controller, but only if the temperature stays around 77F.

The voltage lowers as the temperature rises, dispelling the notion about solar energy that hot days are the greatest times to use solar panels.

A gain of just little more than 18% will occur instead of a gain of 25% when the temperature drops by 10 degrees F.

The advantages of utilizing an MPPT charge controller decrease as the day heats up.

Therefore, even if it is more effective than a PWM, you must carefully consider the circumstance to see whether it suits.

Use Cases for PWM Controllers

You may use a PWM charge controller as long as your 300 watt solar panel and battery are compatible, either 12V or 24V.

More power can be provided by an MPPT controller, but the gain—between 17 and 20 percent—is insufficient to make the expense worthwhile, at least for a small system.

Your solar system won’t often operate at full voltage.

The panels can’t operate at full capacity because to things like clouds, temperature, panel shading, etc.

The differences between a PWM and MPPT are insignificant in small solar PV systems.

The EEEKit Solar Controller is a good option if you want a PWM controller.

When to Employ an MPPT Charger

An MPPT charge controller should be used with solar systems above 400 watts or at 48 volts.

Because the controller will extract the most power from the panels, high voltage PV systems with low voltage batteries will also benefit from an MPPT.

Here’s an illustration.

With a 37 VMP and 8 LMP, your 72 cell, 300W solar panel is equipped.

This produces the following results when connected to a 13V battery and a PWM charge controller:

8 A x 13 V = 104 W

Because the controller reduces the panel voltage to the battery level, you lose more than half of the power from your 300W system.

The system uses the whole 37 VMP if you use an MPPT charge controller:

37 x 8 = 296 watts

Subject to the previously stated environmental parameters of temperature and other environmental considerations, you have almost the entire 300 watts accessible.

Additionally, you may utilize numerous charge controllers if the array is quite big.

These illustrations highlight the benefits of MPPT controllers over PWM controllers for large power PV systems and small-scale solar arrays, respectively.

The effectiveness of the controller, as previously described, is also influenced by the local temperature.

How Many Batteries Will a 300 Watt Solar Panel Require?

Although you should verify the specifications to be sure, most charge controllers are compatible with 12V and 24V systems.

Depending on how you use the system and the kind of controller you installed, you need have a certain number of batteries.

The battery must provide the same amount of electricity that the solar panels do if you want to utilize it as a backup power source.

With five hours of sunlight, a 300W system may produce up to 1500 watts per day (assuming ideal conditions).

1800 watts may be supported by a 150ah 12V battery.

Only 900 watts can be utilized from a lead acid battery before it has to be recharged.

Choose a 12V 300ah battery to achieve 1500 useable watts.

Up to 1800 watts are offered with a 300ah battery.

Your battery bank must be fully charged in order to function as a backup.

Make that the charge controller is the appropriate one and that it is linked to the panels correctly.

If you keep the battery charged, you may use it to power your house or RV at night when there is no solar power in addition to using it as a backup power source.

There is more power accessible the more batteries you have.

However, you should look into the controller’s restrictions.

Refer to the manufacturer’s instructions since certain controllers can only handle a certain quantity of batteries.

Additionally, having too many batteries could strain the solar panels.

Additionally, this can lead to an imbalance since certain batteries might get less power than others.

The answer is to increase the number of solar panels and/or charge controllers.

Are Lithium Batteries Necessary?

Compared to lead acid batteries, lithium batteries discharge power more quickly.

Additionally, they need little maintenance and have a longer lifespan.

They do cost more than lead acid batteries, however.

However, keep an eye on it since lithium battery costs are decreasing.

Lead acid batteries ought to be plenty for a 300 watt setup.

The fact that a lead acid battery cannot be used to its full potential may be compensated for if you have more than one.

However, lithium batteries are an excellent option if you have a high-end system and don’t want to bother with the upkeep.

Lithium, AGM, gel, FLA, or SLA are usually no problem for charge controllers.

To start, read the product instructions since certain controllers could be more suited for some batteries than others.


One of the most important parts of a solar system is the charge controller.

Make careful to get from a respected manufacturer whether you opt to go with a PWM or MPPT charge controller.

There won’t be any issues as long as the solar panels, battery, and controller are compatible.

Published on

Written by Bob Matsuoka
Bob Matsuoka is a blogger and founder of RVing Beginner blog. He has been blogging for over five years, writing about his own family’s RV adventures, tips for people who are interested in buying an RV or taking their family on an adventure by RV.