The main rating of a solar charge controller that you should consider is its Output Current rating.
The Output Current rating or the “size” of the solar charge controller that you need for your 300w solar panel, will mainly depend on 3 factors:
- The voltage of your battery bank
- The voltage of your solar panel
- The type of solar charge controller you’ll be using
After reading this, you’ll be able to determine the size of the charge controller that you need, and you’ll learn about the 2 types of charge controllers and which one you might want to use.
What size charge controller for 300W solar panel?
In general, if your 300W solar panel and battery bank are both rated at 24V nominal, you would need a 15 Amp solar charge controller. If your solar panel is rated at 24V, but your battery bank is only rated at 12V, you would need a 30A MPPT solar charge controller or a 15 amp PWM charge controller.
If your 300W solar panel (or solar array) and battery bank are both rated at 12V nominal, you would need a 30A solar charge controller.
Here’s a table that shows you what size charge controller you’ll need for your 300W based on its nominal voltage, the nominal voltage of the battery, and the type of charge controller:
| Solar Panel’s voltage | 24 V | 24 V | 12 V |
| Battery Bank’s voltage | 24 V | 12 V | 12 V |
| MPPT charge controller’s size (Amps) | 15 A | 30 A | 30 A |
| PWM charge controller’s size (Amps) | 15 A | 15 A | 30 A |
Notice that for a 24V solar panel/12V battery setup, the size of the charge controller depends on the type of charge controller you’ll be using.
This is because MPPTs and PWMs each operate in an entirely different manner, and it becomes more obvious when the voltage of the solar panel is much higher than that of the battery.
So, should you use an MPPT charge controller or a PWM charge controller?
MPPT vs PWM: which should you use for your 300W solar panel?
In my article about the function of solar charge controllers, I explain how both MPPTs and PWM serve a common purpose, which is to protect the battery from overcharging and over-discharging. However, these 2 types of charge controllers operate differently, and although they are more expensive, MPPTs are generally more efficient than PWMs.
A PWM charge controller lowers the voltage from the solar panel(s) by turning ON and OFF, or in other words, by disconnecting and then reconnecting the solar panel, which allows the battery to charge safely. This does protect the battery but results in power losses. Learn more about PWM charge controllers here.
On the other hand, an MPPT charge controller does 2 things:
- First, it maximizes the power production of the solar panel(s).
- Then, it takes that maximum amount of power and transforms it into a lower voltage/higher current power. This allows the battery to charge safely but doesn’t cause any losses in power production.
This makes MPPT charge controllers much more efficient. Learn more about MPPT charge controllers here.
Exactly how much more efficient an MPPT is, compared to a PWM, will depend on the difference in voltage between the solar panel(s) and the battery bank. The higher the voltage of the solar panel(s) compared to that of the battery, the less efficient a PWM becomes, and the more it makes sense to use an MPPT instead.
So, if your 300W solar panel is rated at 24V (nominal), and you’re planning on charging a 12V battery bank with it, use an MPPT charge controller.
If your solar panel and battery are rated at the same nominal voltage, you can use either a PWM or an MPPT. However, an MPPT solar charge controller would still provide better results in terms of power production.
Either way, you can use our MPPT charge controller calculator to find a suitable MPPT, or our PWM calculator to find the PWM that you need.
If you’d like to learn more about the calculations involved in sizing solar charge controllers, the next section explains how each type of charge controller is sized.
How to select a solar charge controller?
How you size a solar charge controller depends on whether you’re trying to size an MPPT charge controller or a PWM charge controller.
How to size an MPPT charge controller?
To select the right size MPPT for your system, simply divide the power rating (in Watts) of your solar panel or array, by the lowest voltage required to charge your battery bank. Then select an MPPT charge controller that has an Output Current rating (in Amps) that is greater than the value you’ve just calculated.
The lowest voltage required to charge the battery is:
- 10.5 Volts if your battery is rated at 12V (nominal)
- 21 Volts if your battery is rated at 24V (nominal)
- 42 Volts if your battery is rated at 48V (nominal)
For example, let’s say you’re trying to charge a 12V-100Ah battery with your 24V-300W solar panel:
MPPT’s Output Current rating (Amps) should be greater than Solar Panel’s power rating (Watts) ÷ Lowest Voltage Required to charge the Battery (Volts)
MPPT’s Output Current rating (Amps) should be greater than 300 Watts ÷ 10.5 Volts
MPPT’s Output Current rating (Amps) should be greater than 28.5 Amps
A 30A MPPT charge controller would be a nice fit for this setup.
How to size a PWM charge controller?
To select the right size PWM, multiply the Short-Circuit current of your solar panel (or array) by a factor of 1.25, then select a PWM charge controller that has an Output Current rating that is greater than the value you’ve calculated.
For example, let’s say you’re trying to charge a 12V-100Ah battery with a 24V-300W solar panel. And let’s also assume that the solar panel has a Short-Circuit current rating of 10.5 Amps:
PWM’s Output Current rating (Amps) should be greater than Solar Panel’s Short-Circuit Current rating (Amps) x 1.25
PWM’s Output Current rating (Amps) should be greater than 10.5 Amps x 1.25
PWM’s Output Current rating (Amps) should be greater than 13.12 Amps
A 15A PWM charge controller would do the job.
However, again, since the voltage of the solar panel is much greater than that of the battery, a PWM would have to disconnect the solar panel 50% of the time to lower its voltage and match it to that of the battery. This means that 50% of the energy generated by the solar panel, which could have potentially made it to the battery, will be lost.
An MPPT would be a much better choice for this setup.
What size cable to connect your 300W solar panel to the charge controller?
In general, you would need a 12AWG copper wire to connect a 24V-300W solar panel to the charge controller. If the 300W solar panel (or array) is rated at 12 Volts, you would generally require an 8 AWG copper wire.
However, if the solar panel is more than 25 feet away from the charge controller, you will be required to use thicker wires to limit the voltage drop between the solar panel and the charge controller.
Read more about this topic here.
