Charge controllers are rated based on how much current they can handle, measured in amps (A). The current produced by your solar panels is determined by their total power and the voltage of your battery bank.
You can calculate the current using the formula:
Current (A) = Power (W) / Voltage (V)
This means, 500W of solar panels using a 12V battery needs a solar charge controller of 40 amps.
500W/12V=41.6A
If you have a 24V battery, you can halve this, and your charge controller will be cheaper too.
500W/24V=20,8A
We will talk about this more later in the article.
Undersize or Oversize?
A solar charge controller has a limit on the current output. If your solar array is bigger than the current output of the charge controller it will limit it’s output.
For example, a 500W solar array has a charging current to the battery of 41.6A (500W/12V=41.6A) you can use a 30A charge controller without damaging it.
You will damage a solar charge controller if you go over the maximum input voltage.
You can also choose to have multiple charge controllers charging the same battery. Read more about it in my detailed guide here.
Undersize
Undersizing means having your solar charge controller working at 100% most of the time.
In my recent guide, I talked about overpaneling your solar charge controller. It discusses why you should have more panels on your charge controller than it can handle. For example, using a 30A charge controller with 500W of solar. You will lose some power during the best times of the day, but it will be more cost-efficient on cloudy and winter days.
Read more about overpaneling your solar system here.
Advantages:
- Cheaper charge controller
Disadvantages:
- You will lose some power on most sunny days
- Might reduce lifespan of charge controller
Oversize
Oversizing means having your charge controller work at 80% most of the time.
Some guides on the internet say you should calculate a 20% safety margin, meaning that you never load the charge controller to its maximum output. For 500W solar and a 12V battery, this would become:
500W/12V = 41.6A
41.6A * 1.2 = 50A
Here you will be using a 50A charge controller.
Advantages:
- You can harvest all the energy
- Charge controller may last longer
Disadvantages:
- More expensive
My recommendation
If you are limited by space for solar panels in an RV or boat, oversize your solar charge controller. If you don’t have space limitations, undersize your solar charge controller.
The Effect of Battery Voltage on the Charge Controller
If you increase your battery voltage to 24 or 48V, then your charge controller will be cheaper. Let’s explore this with an example.
500W/12V=41.6A
500W/24V=20.8A
500W/48V= 10.4A
If we double the battery voltage, we can see that the current gets halved.
I recommend getting as high of a battery voltage as possible. You will also save on wires because they need to be less thick.
Conclusion
the size of the charge controller for a 500W solar panel system depends on the voltage of your battery bank and whether you choose to undersize or oversize your charge controller.
Increasing your battery voltage can make your charge controller and wiring cheaper. Always consider your space limitations and the specific needs of your solar system when choosing a charge controller.
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I’m an off-grid enthusiast. I created this website to give clear and straight-to-the-point advice about solar power. I’m also the author of the book ‘Off-grid solar power simplified‘. Read more about me on my about page, check out my Youtube channel, or send me a message.