When you break it down to the simplest forms, there are basically 2 main components to a solar system not counting the battery. The solar panel and the solar charge controller. While many of you understand the job of a solar panel is to convert sunlight into electricity, not as many of you understand just exactly what the solar charge controller does to turn that electricity to power you can use and store in your batteries. In this article, I will try to answers the questions I have heard the most about them to clarify each one.

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Why you need a solar charge controller?

So what exactly is a solar charge controller? Well, let’s talk quickly about solar panels first. In the case of generating electricity, we are specifically talking about PV, or photovoltaic, solar panels. A PV solar panel is what you would call a ‘dumb’ solid-state device. That means there are no moving parts, no integrated circuits, and no software that monitors or controls what it does. Its primary job is to convert sunlight into electricity. When photovoltaic cells convert sunlight into direct-current (DC) electricity flowing down the wire, it is uncontrolled and the amount it produces is based on the strength of the sun’s rays, the angle of the sun on the panel, and whether if it is filtered by any other objects in between like clouds, leaves or things that create shade. To control and use the DC current produced, a separate device is necessary to intercept these electrons and conform them to be used immediately by an appliance or stored into a battery to be used later. That is what the solar charge controller’s job is. It turns a ‘dumb’ solar panel into a ‘smart’ solar generator.

Solar is one of the best ways to extend that stay at your favorite boondocking spot without needing to run the engine or a generator.

Without a solar charge controller, you risk damaging your electrical components and batteries. On a basic level, for a battery to be charged, the incoming voltage has to be higher than the voltage of the battery. Think about it like water in a pipe. The direction of flow is based on how much water pressure is on each side of the pipe. It will flow from the high-pressure side towards to low-pressure side. In the case of electricity, the voltage of your solar panel needs to be higher than the voltage of your battery in order for electrons to flow into it.

If your camper came ‘pre-wired’ for solar (Which has a plug on the side of your camper that looks something like this), that does not mean you are ready to just plug in a solar panel. It just means you have positive and negative wires tapped into your electrical system. You will still need to control the solar input by using a solar charge controller.

A typical 12-volt solar panel will have an open voltage of between 18-volts and 22-volts. Depending on the strength of the sun, you will get a variable current flowing down its wires at that voltage. A 100-watt panel with an open voltage rating of 18 volts will have a maximum current of about 5.5 amperes. That is too high to go directly into your battery. A solar charge controller will take that power and regulated it down to a much more compatible level for your DC electrical system. Generally speaking, it does that by using a couple of different methods: Pulse Width Modulation (PWM) or Maximum Power Point Tracking (MPPT). Let’s talk a little bit about each one.

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What Is Pulse Width Modulation? (PWM)

This is the easiest and cheapest method to regulate your solar panel’s output to a proper level for your battery. In the simplest form, it controls the voltage coming down the wire by intermittently switching the flow on and off. PWM is how many light dimmers and electric motor controllers work as well. Have you ever seen a video where the light in the scene has a weird flicker? That is because the micro-controller of that LED light is very quickly switching the light on and off to vary its brightness. It is fast enough to be unperceivable to the human eye but can be seen through the shutter of a camera. That is pulse width modulation.

PWM alternates the power on and off to regular the voltage appropriately.

PWM solar charge controllers work like dimmer switches for your solar panels. It is simple and effective but it comes at a cost. Every time it is in the off position, you are basically losing the power that is generated by the panels. So if you are generating 5.5 amperes at 18-volts from your 100-watt panel, it needs to shut off intermittently enough that your charging voltage is brought down to around 14.2 volts but you are still only generating 5.5 amperes of current. So instead of getting your full 100 watts, you are only getting 78 watts (5.5 amperes * 14.2 volts = 78.1 watts).

Another downside of PWM is that it can only work with a very narrow range of voltage outputs. If your panels have too high of an open voltage, they will not be able to switch it off enough to bring the levels down to be compatible with your system.

There is an upside of PWM controllers. For those of your who are budget conscious or just want to give solar a try without completely diving in headfirst, they are very simple devices so they are very inexpensive. It can be as much as 5 to 10 times more expensive for a comparably sized unit. For casual use where you just need some basic solar needs, this all you need.

Renogy is a popular brand for entry level PWM charge controllers like this one

Renogy Adventurer 30A PWM Flush Mount Charge Controller

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What is Maximum Power Point Tracking? (MPPT)

To be able to take advantage of all that power being generated by your solar panel, an MPPT charge controller is what you want. Instead of throwing all of that extra solar production away but switching the power off to get it down to the proper voltage, it takes in that extra voltage and converts it into more power for your battery. So if you are running that 100-watt panel and getting 5.5 amperes of output, instead of just dropping the voltage from 18 volts to 14.2 volts and losing 22% of our watts coming it, it will ‘track’ the voltage and transform it to usable amperes So instead of 5.5 amperes at 18 volts, you will get closer to 7 amperes of current at that new 14.2 volts of charging voltage.

You will pay more for an MPPT charge controller but if you are running a reasonably sized system, you can gain more than 20% of charging capacity. The higher your panel’s open voltage the more loss you will have if you choose to use a PWM controller. To speak in practical terms, with an MPPT charge controller, you will produce more energy with a 400-watt system than you could with a 500-watt system using PWM. It doesn’t just save you money from not having to buy an extra panel to make up the difference, it also saves valuable rooftop space for other uses like roof fans, racks, gear carriers, or whatever else you are into.

MPPT algorithms convert higher voltage solar panel output to the appropriate charging voltage while increasing current going into your battery.

Of course, you will pay a premium for an MPPT charge controller but with the extra energy production, it is well worth the cost. Another benefit of having an MPPT controller is that it is designed to accept a much wider range of input voltage. A typical MPPT controller can accept upwards of 100 volts of input power. Why that matters, you might ask? Well, if you have multiple panels, you can choose to connect them in series. That means you connect the positive of one panel to the negative of the next and on and on. Kind of like how you can connect AA batteries in a string to boost up the voltage in electronics. By doing that, you will bring the voltage up and simplify the wiring so you won’t need a big junction box to combine multiple panels in parallel. You can also use much smaller gauge wires that are cheaper and easier to manage. DC wires are typically rated for much higher voltage than you would have in a camper but as amperes go up, the thickness of the wire will also need to increase. Running higher voltage inputs from your solar panels eliminates that.

Unlike a PWM charge controller, an MPPT controller has more complicated sensors and algorithms inside the microprocessors to make it all work properly. So a higher quality and more reputable brand will make a difference in how it performs. I have some personal favorites to recommend if you choose to go this route.

Victron SmartSolar is my personal favorite MPPT charge controller

Victron’s SmartSolar MPPT charge controller

This is a unit rated for 15 amperes with an input maximum voltage of 75 volts. It will allow 3 to 4 panels daisy-chained in series. Larger units are also available.

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Which one do you need for your build?

If budget was of no concern and you are certain about having a solar system in your build, MPPT is definitely the way to go. A high-quality unit will last many years and have much more capability as your system changes over time. You might only have one panel to start with but as your needs increase over time, being able to daisy-chain the panels in series means you won’t need to drill new or bigger holes on your roof to run more wires. But if you are unsure that solar is right for you and perhaps you are only going to run a small portable panel on the ground to start, getting a cheap PWM is a good way to start. You can plug it directly into your camper’s solar pre-wire setup or just use alligator clips directly onto your battery terminals and start enjoying the benefits of free power from the sun.

Whichever way you decide to go, being able to run your camper from the power of the sun is a great way to extend your next boondocking stay and the best way to keep your camper topped off to prevent premature aging of your battery pack.


PWM solar charge controllers I recommend in order of preference:

MPPT solar charge controllers I recommend in order of preference:

Dan Lin

I am a modern nomad traveling full time around the world with my family since 2008. Currently in Europe living in our DIY Sprinter 4x4 Camper Van. You can find me sharing more about this lifestyle on Instagram or YouTube. For our travel content, check out malimish.com.


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