How to Select a Solar Charge Controller for RVs & DIY Campervan Conversions

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A solar charge controller is an essential component for getting the best performance from your camper solar setup.

But not all solar charge controllers are the same.

There’s different types, sizes and features, each designed to best suit off-grid solar setups.

It’s easy to get lost in the technical detail and end up buying an inferior product for your setup.

This guide will help you avoid this so you can buy the best solar charge controller for your campervan conversion or RV and get the best performance from your panels.

We’ll show you some of the best solar charge controllers on the market today, explain the basic functions of the component, the different types of solar charge controllers and how to select the best charge controller for your van lifestyle.

And as an added bonus, we’ve included an easy to use calculator so you can choose the right size RV solar charge controller for your setup. 

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What is a solar charge controller?

The aim of the game is to harvest the sun’s clean, renewable energy in the form of sunlight and use it to charge deep cycle leisure batteries.

The solar panels take care of the harvesting. 

A single solar panel is made up of multiple cells. Because the sunlight hitting them is sporadic and uneven, they don’t all receive the same amount of sunlight at the same time.

This results in the panels producing a wide range of energy, with the potential to fluctuate multiple times every second and spike often. 

Depending on the type, batteries need something between 12.6 volts to 14.6 volts to charge.

Anything less and they won’t charge. Anything more and they’ll start to cook, destroying the battery.

So connecting the batteries directly to the solar panels will result in damage to the batteries.

This is where the solar charge controller steps in, taking over the entire charging process.

As its name suggests, its primary function is to control the solar charge, protecting the battery from harm.

An RV soaking up the free renewable energy

How does a solar charge controller work?

To prevent overcharging the RV or campervan batteries, the solar charge controller regulates the voltage passed to the battery from the panels.

It works by looking at the battery voltage and regulating the battery charging in response.

Batteries take a different voltage for various charging states.

For example, a relatively flat battery may take as much as 14.4v until it’s 90% charged.

Then it needs a lower voltage to trickle charge, maintaining it at a peak state.

The solar charge controller manages this dynamically, increasing or decreasing the charging current and voltage for the given state.

Do I need a solar charge controller in my RV?

Typical RV and camper solar setups need to have a solar charge controller installed.

Applying the random and chaotic voltages from a solar panel directly will permanently damage the battery. 

Not exactly what you want for your DIY campervan conversion or for a comfortable van life.

The only time you might get away without a solar charge controller is if the solar panel is really small panel – less than 5 watts. 

But that’s so small it’s virtually hand held size so not upto providing the energy demands of van life.

In short, if you want a camper solar set up, you need to install a solar charge controller.

Types of solar charge controller

There are 2 types of solar charge controllers for your campervan:

  • Pulse Width Modulation (PWM)
  • Maximum Power Point Tracking (MPPT)

PWM Solar Charge Controllers

A PWM is a simple device. 

Designed to handle a nominal 12v (or sometimes 24v) solar panel array, a PWM won’t cope with the large voltages of multiples panels wired in series.

The total nominal voltage of the solar panels must be the same as the voltage of the battery bank with a PWM controller. 

So a 12v PWM charge controller must be paired with 12v solar panels and a 12v battery setup.

Therefore, when using multiple panels in a solar array with a PWM charge controller, they must be wired in parallel.

A PWM controller takes the power from the panels and passes it to the batteries, depending on the charge state of the battery.

The maximum amps of the solar array cannot be exceeded.

Let’s say a battery is sitting at 12.6v. 

With a 100 watt solar panel rated at 17.9 volts and 5.72 amps the maximum watts you can pass to the battery at that time is 12.6v @ 5.72a giving us 72 watts.

(Ohms Law states watts = volts x amps)

As the battery level increases, the PWM charge controller becomes more efficient. A PWM is at its most efficient when the battery is nearly full. 

At 13.4v however, it can still only draw on about 75% of the watts available on the roof.

13.4 volts @ 5.72 amps is 77 watts.

So a PWM isn’t especially efficient, even at its peak.

The Pros & Cons of PWM Solar Charge Controllers

Solar panels beside an RV

Many campervan converters and RVers turn their back on PWM charge controllers in favour of MPPT but there is still a place for them.

If you intend living in a van off-grid for anything more than a few days at a time, and depending on your campervan electrics, a PWM charger probably isn’t the right choice for you.

It’s simply not efficient enough at extracting the most energy from the solar panels so you’d need more panels to make up for it. If you have space on your roof for them.

But the more panels you have, the more evident those inefficiencies become.

In short, if you can’t manage without your fridge and lights, invest in an MPPT controller.

On the upside though, PWMs are incredibly cheap.

A PWM charge controller to cope with a 300w solar set up will cost in the region of £35 | $50 compared to a £110 | $150 price tag for the MPPT controller for a similar size set up.

PWM charge controllers are most efficient when the battery is nearly full. 

So if you go off grid for only a few days at a time, setting off with full batteries and in fair weather, a PWM controller may be all you need.

MPPT Solar Charge Controllers

An MPPT solar panel charge controller is a more sophisticated device than a PWM.

It can handle much higher voltages so wiring the panels in series is the best configuration.

To explain how an MPPT controller works we need more maths I’m afraid.

Let’s consider 2 x 200w panels wired in series. 

Each panel has an amp rating of 5.72 and an optimum operating voltage (Vmp) of 17.9v. 

The battery is sitting at 12.6v.

Those panels are putting out 35.8 volts and 5.72 amps. The MPPT controller knows it can only send 12.6v to the charging battery.

But here’s where the clever bit comes in. Rather than waste the excess volts from the panels, the MPPT controller converts them into amps. 

Let’s look at the numbers:

35.8 volts is converted down to 12.6v (35.8 ÷ 12.6 = 2.84)

5.72 amps is converted up by the same factor to 16.25 amps (5.72 x 2.84 = 16.25)

So power into the MPPT is the same as the power out.

In reality, MPPT controllers are between 94-98% efficient, but the principle of the maths is the same.

The upshot is an MPPT controller can increase performance by upto 30% over a PWM. 

Not too shoddy eh?

The Pros & Cons of MPPT Solar Charge Controllers

Solar panels on the roof of an RV

The benefits of MPPT charge controllers are pretty obvious.

With such high performance, they can keep batteries charged with fewer solar panels. 

Because the controller can handle larger voltages, wiring the solar panels in series is an option. 

Check out our complete guide to wiring your solar panels in series or parallel. It covers mixed panels too and includes an interactive calculator to find the most efficient setup for your array.

In summary though, a series array will provide a high enough voltage to charge the batteries for longer than if they were wired in parallel.

All of this adds up to the opportunity to stay off grid for longer. We’ve been living in a van in South America for almost 3 years now.

The last 6 months we’ve been in Patagonia, throughout the southern hemisphere winter and not driven anywhere – there’s a global issue preventing us from travelling apparently.

We’re not hooked up, we have 320w panels wired in series with an MPPT controller and with careful management, we’re doing just fine.

The only downside of the MPPT is its hefty price tag, about 3 or 4 times the price of a PWM controller for a similar size system. 

However, to get the same battery charge from a PWM charger on a 300w solar setup, you’d need an extra panel and more cable connectors because you’d have to wire in parallel. 

The upshot?

An MPPT solar panel charge controller works out more or less the same, or maybe a little more cost effective.

How to connect a Solar Charge Controller to Solar Panels

Fitting solar charge controllers in an RV is a relatively simple task

Fit a solar charge controller as part of the camper solar setup and after fitting and wiring the panels.

Connecting the solar charge controller to the solar panel is pretty straight forward.

The cables should be connected in the correct order to avoid harm or damage.

Including fuses in the installation is another safety feature too.

We have complete 12v solar panel wiring diagrams for 100w – 800w systems if you want to see the detailed design.

And we have a step by step guide on how to install solar panels on RVs, campervans and motorhomes

It includes the installation of the solar charge controller so is the best place to see how and when to connect it to the solar array.

How to use a Solar Charge Controller

DIY Campervan Solar System

Each solar charge controller model is slightly different in terms of its features and operating controls, so always read the instructions manual first.

Regardless of model though, the basic functions operate in a similar way.

Once the controller is connected to the solar array, the panels will produce an electrical supply. 

This allows you to configure the controller to support the specific battery types and charging profile of your battery.

Follow the manufacturers instructions carefully.

Once the solar charge controller is set up and the batteries are connected, the solar charge controller usually needs no further intervention or maintenance.

Top tip | Keep the manufacturer’s manual in your RV or campervan. It helps to refer to it when monitoring the performance of your solar setup.

What to Look for When Buying a Solar Charge Controller

Solar charge controllers for RVs

Charge controllers, specifically MPPTs are an investment so its important to select the best one for your use.

Solar charge controllers come with different efficiency ratings and a variety of extra features.

Here’s the most common features and technical terms to look out for when buying a solar charge controller:

Type | Refers to MPPT or PWM.

Supported battery technology | The type of battery (FLA, AGM, Gel or Lithium) the charge controller can work with. 

Efficiency rate | The higher the percentage, the more wattage the solar charge controller can extract from the panel. The higher the better.

Battery output voltage | Most solar charge controllers cover a range from 12v to 48v. Most RVs, campervans and motorhomes run on 12v systems, while 24v battery setups are less common. 

Maximum input voltage | The highest voltage the solar charge controller can accept from the solar panels. Determine what the highest possible input voltage could be for your solar panel configuration including future scalability. Remember, wiring panels in series provides a higher voltage.

Maximum current output | The highest current (amps) the charge controller can send to the battery bank. If you size this too small, you waste the energy your solar panels harvest.

LOAD or LVD output | Allows small devices to be charged directly from the solar charge controller instead of the battery (a waste of time in our opinion).

LED screens | For displaying system operation information and error codes (helpful because without it you’ll need battery monitoring displays so you can keep an eye on the health of the system).

Bluetooth module | For remote control management and monitoring.

Safety features to prevent overcharging, overload, short-circuits, reverse polarity, reverse current and electric arcs.

What size charge controller do I need?

Choosing the size of the controller requires some maths to help identify the correct sized controller.

There are 5 factors to consider:

  • How many solar panels in the array
  • How many watts in the solar panel array?
  • The open-circuit voltage (Voc) of each panel
  • The short-circuit current (Isc) of each panel
  • What battery bank voltage will you use?

With this information you can calculate the minimum size solar charge controller you need for your setup.

Use our solar charge controller calculator below to find the right size for your set up.

If you don’t know the open-circuit voltage or short-circuit voltage of the panels, leave these values set to zero so we can use an estimated value for your panels.

The calculator also recommends an MPPT and PWM charge controller to meet your needs.

For more help and information, check out the FAQs on our solar charge controller calculator post here.

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We're handy DIY camper van convertors, not professional coach builders, technicians or electricians.

While we're happy with the techniques and products we use, be sure to check all methods and materials you use are compatible with your equipment and abilities.

Regardless of what we recommend, consult a professional if you're unsure about working on your RV, campervan or motorhome.

Any task you perform or product you purchase based on any information we provide is strictly at your own risk.