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What is a Solar Charge Controller?

The Charge Controller takes the power made by the solar panels and transform the ‘solar panel power’ into a form of power that the batteries can use.

Quick note before we get started.  This is just one part of a overarching “How to Install a DIY Camper Van Electrical System” series.  If you’ve just stumbled on this article directly without seeing that, there are likely some things we’ve already covered.  If you want to check out that step by step guide, you can do that here:

Also, we have interactive solar wiring diagrams that are a complete, A to Z solution for teaching you exactly what parts go where, what size wires to use, fuse size recommendations, wire lug sizes, and all kind of other stuff to help save you time and frustration.  You can check that out here:

Finally, for this blog post you’re reading right now, we have a calculator that will help you choose a charge controller. I HIGHLY recommend reading this post to truly learn how to use the calculator, but if the calculator is all you’re looking for, you can download that HERE.

How does the Charge Controller Work?

Solar panels typically put out a voltage that is too high for batteries to use. If you have your solar panels wired in series like I recommend, you could possibly have over 100 volts coming out of the solar panels. If you connected your 100 volts from the solar panels directly to the battery, it’s not going to work. The Charge Controller regulates the voltage from the solar panels back down to the 12.6 – 14.6 volts that the batteries can store/use.

Charge Controller regulates the voltage from the solar panels.


There are two main types of charge controllers. They are MPPT and PWM. This blog post is a crash course in solar design and getting into the specifics of the differences is out of the scope of this blog post. Here’s what you need to know regarding MPPT vs PWM charge controllers MPPT is the newer, more efficient technology. From here on out, any time I talk about charge controllers, I will only be talking about MPPT charge controllers as I want to guide you to build a high-end, expandable solar setup.


One of my favorite series of charge controllers is the Victron BlueSolar MPPT Charge Controller. If you’ll notice, there are MANY different sizes of charge controllers:

  • Victron SmartSolar MPPT 75 | 10
  • Victron SmartSolar MPPT 75 | 15
  • Victron SmartSolar MPPT 100 | 15
  • Victron SmartSolar MPPT 100 | 20
  • Victron SmartSolar MPPT 100 | 30
  • Victron SmartSolar MPPT 100 | 50
  • Victron SmartSolar MPPT 150 | 35
  • Victron SmartSolar MPPT 150 | 45
  • Victron SmartSolar MPPT 150 | 60
  • Victron SmartSolar MPPT 150 | 70
  • Victron SmartSolar MPPT 150 | 85
  • Victron SmartSolar MPPT 150 | 100
  • Victron SmartSolar MPPT 250 | 85
  • Victron SmartSolar MPPT 250 | 100


Lets use the Victron SmartSolar MPPT 100 | 30 for example. The 1st number, 100 means the maximum input voltage the controller can handle. In other words, the Victron SmartSolar MPPT 100 | 30 can handle a max of 100 volts coming from the solar panels into the charge controller. The 2nd number, 30, represents the max amount of amps the controller can output going INTO THE BATTERIES.


Let’s say, for example, you have 4 x 100 watt solar panels with the following stats.

EACH 100w solar panel has an Open-Circuit Voltage (Voc) of 21.6 volts. and an Optimum Operating Current of 6.72 Amps. Those are the only two numbers we are concerned about for now. I generally recommend just wiring all of your solar panels in series for simplicity and efficiency sake. Which means: Those 4 x 100 watt solar panels get wired together like this:

Since they are wired in series, the voltages get ADDED together for a total of 86.4 volts. (Open-Circuit Voltage (Voc) of 21.6 x 4 panels) The amps on the “upstream” side of the 100w solar panels remains 6.72 since in series, the voltages get added and the amps stay the same.

So, the 86.4 volts is under the safe threshold of the 100 max volts of the Victron SmartSolar MPPT 100 | 30 solar controller.

100 is the first number. What about the 2nd number, 30?

The 30 in the Victron SmartSolar MPPT 100 | 30 is the MAX resulting amps AFTER the solar controller has worked it’s magic. We need to do some math to determine the amperage. Here are the things we know:

  • We have 4×100 watts of solar panels totaling 400 watts of solar.
  • Assume batteries are 12.6v
  • Amps = Watts / Volts

This means, that at 400 watts and 12.6v we can expect up to 31.74 amps coming out of the solar controller.

400 watts / 12.6 volts (Battery) = 31.74 amps coming out of the charge controller.

Now, we are talking about that Victron SmartSolar MPPT 100 | 30, we have to compare that 2nd number, 30.

31.74 amps is a bit over the 30 amp threshold. BUT…

Solar panels rarely put out their full wattage. AND…

In the Victron SmartSolar MPPT 100 | 30 manual, they say their controller is good for solar arrays up to 440 watts:

It’s always good practice to trust manufacturer specs and recommendations.

AND… If you happen to go ‘over’ on your Amperage, it’s not that big of a deal in terms of damage. It’ll just be lost power that the controller won’t convert.

So, basically, the Victron SmartSolar MPPT 100 | 30 is pretty perfect for those 4 x 100 solar panels.

But what if you like playing it safe? What if you want some wiggle room? Great! Size up to the Victron SmartSolar MPPT 100 | 50. Sure, it’s a little more money, but if it’s worth your piece of mind to have an extra 20 amps available to you, go for it.

Now, Why would you want wiggle room or safety margin? Let’s talk about temperature

Solar Controller vs Temperature

DID YOU KNOW… As temperatures drop, solar panels actually put out MORE power.

Totally honest though, the math gets messy, SO I made a calculator that you can input all of the values for your setup so YOU can see how temperature affects your solar panel setup AS WELL AS will give you a recommendation on what solar controller you need taking solar panel temperature into account.

Access that calculator (and it’s instructions) for free, here:


Now that you’ve gone through the Charge Controller Wizard, you’ve gotten a very helpful piece of information:

Based on the inputs, you now have a recommended charge controller size that takes temperature into account.

Sure, it says “Solar Controller 100 | 40” which is formatted like the Victron charge controllers, but you can, ultimately use any charge controller brand you like. The premise is mostly the same.

Now… a Victron SmartSolar MPPT 100|40 doesn’t exist. All you have to do, is simply round up to the next nearest available size, which would be the Victron SmartSolar MPPT 100|50.

*This calculator will give you some wiggle room and a buffer for the panels you have input. If you feel you want to round down a size, ALWAYS listen to the minimum specs that the manufacturer has made.*

Confused? Don’t want to do math?

No problem. I’ve got you…

Here is a list of solar panel combinations with my recommended Victron solar controller size. I’ve picked these combinations assuming you’ll never be somewhere with a daytime high of colder than -10 degrees Fahrenheit.

  • 100 Watt Solar Panel Options:
    • 100 watts: Victron SmartSolar MPPT 75 | 10
    • 200 watts: Victron SmartSolar MPPT 100 | 20
    • 300 watts: Victron SmartSolar MPPT 100 | 30
    • 400 watts: Victron SmartSolar MPPT 100 | 50
  • 175 Watt Solar Panel Options:
    • 175 watts: Victron SmartSolar MPPT 100 | 20
    • 350 watts: Victron SmartSolar MPPT 100 | 50
    • 525 watts: Victron SmartSolar MPPT 100 | 50
    • 700 watts: Victron SmartSolar MPPT 150 | 70
    • 875 watts: Victron SmartSolar MPPT 150 | 100

Now that you know what kind of charge controller is compatible with your solar panels, it’s time to learn how to choose an inverter for your DIY Camper setup.  Check that out here:

Everything that you are learning here is put to use in our FREE Interactive Solar Wiring Diagrams.  If you haven’t yet, check them out as they are a complete solution for a camper van electrical system.  Check them out here:

Remember, this is just one part of a full camper van electrical educational series.  To see all of the individual guides, click here:

Finally, If you found this guide helpful, It’d truly mean the world to us if you’d share it with somebody who can use it, pin it to pinterest for later reference, or share it to a facebook group when somebody has a question about this subject.  Click the bubble in the lower right corner to subscribe to be notified of future updates and as always, leave any questions you’ve got in the comments below.