What Size Charge Controller For a 300 Watt Solar Panel?

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A 300 watt solar panel needs a charge controller to store power in the battery bank. If the controller is not properly matched with the panel it will not work, so knowing how to calculate the size is important. Fortunately the steps are really easy.

A 12V 300 watt solar panel requires a 30A charge controller, provided the controller is compatible with the system battery voltage. Most 30A charge controllers are designed to work with 12V and 24V batteries, but 48V batteries require a larger one.

How to Calculate Charge Controller Size

Charge controllers are measured in amps. The basic rule is the controller amp rating must be higher than the amps of the solar panels or solar array. The formula is:

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

So with a 12V 300 watt solar panel, the formula looks like this:

300 watts / 12V = 25 amps + 20% = 30

You need a 30 amp charge controller for this system. Our choice is the Renogy 12V/24V 30A MPPT Solar Controller. This controller works with 12 and 24V systems as well as AGM, gel and lithium batteries.

Some experts recommend adding 25% instead of 20%. If you went for 25% the result would be 31 amps, or a 35A controller. The choice is yours, though a 20% safety margin is usually enough.

This formula assumes the system is running on 12 volts. If you are using a 24V system you can still follow these steps though the results will be different.

For instance, if you connect 4 x 300W 24V solar panels in a series, you would need a 60A charge controller.

4 x 300 = 1200

1200 / 24 = 50

50 = 20% = 60

What are VMP and LMP in Solar Panels?

There are two numbers you need to check on the solar panel specifications: the VMP (voltage maximum power) and the LMP (maximum current). The VMP for 300 watt solar panels made for 12V is usually 18V and the max current at 5.7A.

So technically, a 12V solar panel runs higher than 12V, but that is also the case with batteries, which charge higher than their voltage. Higher rated systems may have a 37-40 VMP and 8A max current, so check your panel specs first. In the following section you will see why these numbers determine how much power you can get from a solar system.

What Charge Controller Type Should I Use?

There are significant differences between a PWM and MPPT charge controller, but the most important in this case is how they handle power coming from the solar panels.

A PWM charge controller is ideal for a 12V or 24V 300 watt solar panel, provided the battery voltage is similar. If the solar panel voltage is much higher than the battery, use an MPPT charge controller.

For example, a solar panel is running at 18V VMP and has a 5.2 LMP. A 12V battery is connected to the system and is charging at 13V (the voltage can range from 10.8 to 14.4V).

With a PWM charge controller the system draws 67.6 watts (5.2A x 13 volts = 67.6). This is how much power the PWM controller will pull from the solar panel as long as it stays at 13 volts. In short, a PWM controller will match the solar panel voltage to the battery, dropping from 18V to 13V.

If you use an MPPT charge controller, the system will draw the full 18V from the panel. So that means 5.2A x 18V = 90 watts. That is a 25% increase over a PWM controller, but this will only happen if the temperature remains around 77F. As the temperature climbs the voltage drops, which dispels the solar power myth that solar panels work best during hot days.

A drop of 10 degrees F can lead to a 5% voltage drop, so instead of a 25% gain it will be just under 18%. The hotter the day gets, the fewer benefits you will get from using an MPPT charge controller. So while it is more effective than a PWM, you have to look at the situation carefully if it fits.

When to Use a PWM Controller

As long as your 300 watt solar panel and battery are matched, either 12V or 24V, you can use a PWM charge controller. An MPPT controller can provide more power, but the increase -17% to 20% – is not enough to justify the cost, at least for a small system.

In many cases your solar system will not run at maximum voltage. Factors like clouds, temperature, shading on the panel, etc. prevents the panels from running at full power. In small solar PV systems the difference between a PWM and MPPT are small enough to be negligible. If you prefer a PWM controller we suggest the EEEKit Solar Controller.

When to Use an MPPT Charge Controller

Solar systems above 400 watts or at 48V should use an MPPT charge controller. High voltage PV systems paired with low voltage batteries will also benefit from an MPPT because the controller will draw the maximum power from the panels.

Here is an example. You have a 72 cell, 300W solar panel with a 37 VMP and 8 LMP. If you connect this to a 13V battery and a PWM charge controller, you get the following:

8A x 13V = 104 watts

You lose more than half the power of your 300W system because the controller pulls the panel voltage down to the battery level.

If you have an MPPT charge controller, the system draws on the full 37 VMP:

37 x 8 = 296 watts

You have almost the full 300 watts available, subject to temperature and other environmental factors as mentioned earlier. And if the array is really large, you can use multiple charge controllers.

These examples show why MPPT controllers are the best for high powered PV systems while PWM controllers for small scale solar arrays. The temperature in your location also plays in role in determining how effective the controller works as explained above.

How Many Batteries Do I Need For a 300 Watt Solar Panel?

Most charge controllers are compatible with 12V and 24V systems, though you should check the specs to be sure. How many batteries you should have depends on how you run the system and the type of controller you installed.

If you are going to use the battery as a backup power source, it must provide the same amount of power you use from the solar panels. A 300W system can provide up to 1500 watts a day with 5 hours of sunshine (assuming ideal conditions).

A 150ah 12V battery can hold up to 1800 watts. If this is a lead acid battery only 900 watts can be used before it has to be recharged. To get 1500 usable watts, go with a 12V 300ah battery. With 300ah there is up to 1800 watts available.

To serve as a backup your battery bank needs to be fully charged. So make sure it is connected to the panels properly and that you have the right charge controller. Keeping the battery charged allows you to use it not only as a power backup but also to run your home or RV at night when there is no solar power.

The more batteries you have, the more power available. However you should check the limitations for the controller. Controllers can handle only a specific number of batteries so refer to the manufacturer guidelines.

Also, too many batteries could put a strain on the solar panels. This might also cause imbalance as some batteries may receive less power than others. The solution is to add another charge controller and / or add more solar panels.

Do I Need Lithium Batteries?

Lithium batteries have a higher discharge rate than lead acid batteries. They do not need any maintenance either and last longer. However they are more expensive than lead acid batteries. The prices for lithium batteries are going down though, so keep an eye on it.

For a 300 watt system, lead acid batteries should be enough. If you have more than one, it could make up for the fact that you cannot use a lead acid battery to full capacity. But if you have a high end system and don’t want to deal with the maintenance, lithium batteries are a good choice.

Most charge controllers run fine with lithium, AGM, gel, FLA or SLA. However some controllers may be more optimized for certain batteries than others, so read the product guide first.


The charge controller is one of the most critical components in a solar system. Whether you decide to go for a PWM or MPPT charge controller, make sure to buy from a reputable manufacturer. As long as the solar panels, batteries and controller match, there will be no problems.