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How to Properly Size the Solar Charge Controller for Your RV

How to Properly Size the Solar Charge Controller for Your RV

A solar charge controller is a vital part of any RV’s solar system. But, when planning to install solar on your RV, how do you size the solar charge controller(s)? This is important information, but fortunately, the process isn’t difficult as long as you have the details.

In this post, we’ll tell you exactly how to calculate solar charge controller size for your RV’s solar system.

What Is a Solar Charge Controller?

A solar charge controller is an essential component of your RV’s solar system. It sits between your solar panel (or array of solar panels) and your battery bank. Its purpose is to regulate the flow of charge from the panels to the batteries, helping to ensure your batteries are safely charged and not damaged.

The rate of charge and the volume of energy going into your battery bank are regulated by your solar charge controller. Typically, it can be configured to provide the proper charging profile for your battery’s chemistry depending on what type of batteries you have – flooded lead-acid, AGM (absorbed glass mat), lithium (LiFePO4), etc.

Your solar charge controller also acts as an electrical backflow preventer, stopping the energy that’s stored in your batteries from flowing backward into the solar panel when the sun goes down. Without it, this would drain the energy stored in your batteries and could damage the panels.

Photo of a solar charge controller

A solar charge controller acts as a voltage regulator and also as a one-way valve to energy in your batteries from flowing backward into the solar panel when the sun goes down. It’s an essential piece of your RV’s solar system.

So, a solar charge controller regulates the energy going into the battery bank AND prevents the stored energy current from flowing in the other direction (back to the solar panels) when the battery is fully charged and darkness falls. The solar charge controller is as essential to your RV solar system as the solar panels and batteries are.

Is a Solar Charge Controller Necessary?

Yes. If you have an RV solar system, you absolutely need a charge controller. Solar panels alone are not capable of limiting or regulating their voltage output to your batteries. So, without a charge controller, your battery bank is likely to be damaged from overcharging.

In theory, you can connect a very small solar panel (no more than 5-10 watts) directly to a battery to trickle-charge it for maintenance purposes while in storage. However, any RV solar system of any larger capacity needs a charge controller between the solar panels and the battery bank.

What Types Of Solar Charge Controllers Are Available?

There are two different types of solar charge controllers – PWM and MPPT.

PWM Charge Controllers

PWM stands for “Pulse Width Modulation.” To charge your battery bank, a PWM charge controller uses a series of short pulses (as opposed to a steady stream) of power. It’s constantly checking the status of your batteries and uses that information to determine the timing and size of the energy pulses it sends.

If your battery bank is discharged, a PWM charge controller will send out long, continuous pulses of energy to your batteries. However, if your battery bank is fully charged and there’s no load on the system (you aren’t running anything requiring power), the PWM charge controller will send out only a tiny pulse every few seconds.

A graph showing different percentages of pulse width modulation

The graph above shows examples of different pulse width modulation percentages. Lower duty cycle percentage would mean less time charging.

A PWM solar charge controller is the less expensive option, however, it’s only suitable for a low-power solar system with fewer, lower-output panels. And, typically, only for panels that are wired in parallel, not series. They’re also best for trickle-charging a battery for maintenance when it’s not being used. PWM charge controllers have very low voltage loss, so if your devices and appliances are turned off, your battery power is not being consumed.

MPPT Charge Controllers

MPPT (Maximum Power Point Tracking) solar charge controllers are electronic DC-to-DC converters that take the high-voltage power output supplied by a solar panel and convert it to a lower voltage, so your battery bank charges safely and efficiently. An MPPT solar charge controller constantly analyzes your solar panels and determines the exact voltage required to maximize the panels’ power output to your batteries. This voltage changes depending on the amount of light going into the panels, the ambient temperature, and other factors.

MPPT charging graph

The graph above shows an example of the maximum power point for a sample panel. An MPPT charge controller monitors how that point moves with changing conditions (caused by variations in light, temperature, etc) and determines the optimal voltage for the panels to operate for maximum power output.

For this reason, MPPT charge controllers allow more efficient solar panel performance (by up to 30% when compared to PWM charge controllers). They allow the panels to operate at their optimal (higher) voltage for the current conditions… and convert that extra voltage into additional charging amps.

If you have a larger solar array, you’ll want to opt for an MPPT solar charge controller. This will allow you to wire multiple solar panels together in series, which increases the voltage output of the combined array. This is a benefit for several reasons:

  1. Higher voltages and lower amps allow you to use thinner, lighter, less expensive, and more flexible wiring to connect your panels to the controller.
  2. The same higher voltage, lower amperage flow results in less voltage drop between your panels and the charge controller, again improving the overall efficiency of the system.
  3. Because an MPPT charge controller can take that extra voltage and convert it to additional amps to charge your batteries, your batteries will charge faster.

Information You’ll Need To Size The Solar Charge Controller

When you’re putting together a solar system for your RV, there are several things you need to consider. You first have to determine how much solar you need for your RV, and then whether to wire your solar panels in series or parallel. You also need to know how to calculate solar charge controller size as it relates to your specific system. In other words, you need to know what amp and voltage ratings your solar charge controller needs to be able to handle.

For a down-and-dirty calculation, you can start with the total watt output of your solar array (# of panels) x (watt rating of a single panel). Then, divide that number by the voltage of your battery bank, and that will tell you (roughly) the total charging current your solar controller will need to be able to handle.

As an example, a 1,200W solar array paired with a 12V battery bank would mean:

1,200 Watts ÷ 12V = 100 Amps

So, the system would need a solar charge controller that could handle at least 100 Amps. But this simple calculation isn’t the most accurate and could result in a system that underperforms. Or, worst case scenario, could cause damage to some of the equipment.

For a more accurate plan, you’ll want to gather information about your whole solar electrical system. What’s the voltage of your RV’s battery bank (12V, 24V, or even 48V)? What’s the voltage output of your solar panel array? (This will depend on how your panels are wired, more on that in a bit.) And what’s the total current output (amps) of the solar array? (Again, this will depend on how the panels are wired.)

Split screen showing an RV with a small solar array on top and a large motorhome with a large solar array on the bottom

Whether you have a small solar array or a large array like the one we had on our Newmar Mountain Aire, The general rule of thumb is to select a solar charge controller with an amp rating that’s 25% higher than the total peak power amperage of your solar array.

Before we get into the solar charge controller sizing, let’s go into more detail about the pieces of information you need to have on hand first.

NOTE: to keep things simple, and to make sure your solar array operates at maximum efficiency, we’ll assume that every panel in the array has the same specs. While you CAN mix and match panels of different outputs, etc, it can result in a less-efficient array if not handled correctly and is not an ideal scenario.

System Battery Voltage

The current RV industry standard is 12 volts, so that’s the most likely situation you’re in. But your battery bank could be 24V, 36V, or even 48V. If it’s one of these less-common voltages, you’d probably know (i.e. you installed, or had installed, a higher voltage battery system). If you don’t know, you can use a multimeter  to measure the voltage across the main cables coming off the battery bank (they’ll likely be the biggest/fattest cables attached to the bank). Do this with the battery fully charged, and no large loads running.

Solar Array Voltage

For this, you’ll need to check the specs for your solar panels. This information can often be found on a label on the back of the panels, but that may be hard to see if they’re already mounted to the roof (or haven’t been purchased, yet). In that case, you should be able to find the same info in either the owner’s manual for the panels, their spec sheet, or the manufacturer’s website.

Specs for our Rich Solar Mega 250 Watt solar panels

This is the spec label for the Rich Solar Mega 250 solar panels that MYT Solar installed for us on our new trailer (more details about that project coming up soon).

There will be two voltage numbers listed:

  • Maximum Power Voltage (Vmp): the operating voltage at which the panels will output their maximum power, in standard conditions (temperature, amount of ambient light, air density, etc)
  • Open Circuit Voltage (Voc): the maximum voltage the solar panels will produce when there is no load on them (i.e. if you measure across the open ends of the panel’s electrical connections with a multimeter)

Open Circuit Voltage (Voc) is the higher of the two numbers, so you should use this when sizing your solar charge controller.

Calculating Solar Array Voltage: Panels Wired In Parallel

If all of your solar panels are wired in parallel, the voltage of the whole array equals the Open Circuit Voltage (Voc) of a single panel (since when wired in parallel, the array voltage remains consistent while the current (amps) from each panel gets added together).

Using the data from the Rich Solar Mega 250 panel spec sheet above, if we had an array of 3 panels wired in parallel, the array voltage would be 22.8V (the Voc of a single panel).

Calculating Solar Array Voltage: Panels Wired In Series

If your solar panels are wired in series, the voltage of the whole array equals the number of panels x the Open Circuit Voltage (Voc) of a single panel. Wiring in series results in the voltage of each panel in the series being added together, while the amperage output remains consistent.

Again, using the Rich Solar panels from above, 3 panels wired in series would result in an array voltage of 68.4V (= 22.8V (Voc ) x 3 panels).

Solar Array Current (Amps)

Same as with the solar array voltage, you’ll need to check the specs on your current (or planned) panels. There will be two numbers that are typically reported:

  • Maximum Power Current (Imp): the output current (amps) when the solar panel’s power output is the greatest, in standard conditions (temperature, amount of ambient light, air density, etc)
  • Short Circuit Current (Isc): the current (amps) passing through the solar panel when the voltage across the panel is zero (i.e. when the solar panel is short-circuited)

The Short Circuit Current (Isc) is usually the highest, so you’ll be using that for calculating your array’s output current.

Calculating Solar Array Current: Panels Wired In Parallel

If the panels in your solar array are wired in parallel, the maximum output current of the whole array equals the number of panels in the array x the Short Circuit Current (Isc) of a single panel (again, when panels are wired in parallel, the array voltage remains consistent, but the current from all of the panels gets added together).

Using our same panels as above, the solar array current of 3 panels wired in parallel would be 41.1 Amps (= 13.7 Amp (Isc) x 3 panels)

Calculating Solar Array Current: Panels Wired In Series

Panels wired in series will have a maximum output current equal to the Short Circuit Current (Isc) of a single panel in the array. For an array of 3 of our sample panels, the solar array current of that array wired in series would be 13.7 Amps (the Short Circuit Current (Isc) of a single panel).

How Do You Size a Solar Charge Controller for an RV?

Now that you have all the data/specs for your solar array and RV batteries figured out, you can proceed with determining the size solar charge controller you need. The process is slightly different, depending on whether you’re using a PWM or MPPT controller.

Sizing a PWM Solar Charge Controller

With a PWM charge controller, you’d start by making sure it was rated for the same voltage as your battery bank. Then, you’d need to match the current output of your solar array. PWM controllers only regulate the voltage being sent to the batteries by pulsing the output (cycling between on and off), but they can’t actually regulate the output current. Instead, they just allow the array’s current through to the batteries.

A Xantrex C60 PWM solar charge controller

Here’s an example of a PWM charge controller, a 60-amp model from Xantrex.

As a result, you need to be sure that your PWM controller is rated to handle sufficient amperage to keep it from being damaged. The general rule of thumb is to select a PWM controller with an amp rating that’s 25% higher than the total peak current output of your solar array. The reason for the additional 25% is that in certain conditions your solar panels may produce more than their rated output. With 25% additional charging capacity, you’re prepared for those conditions.

So, from our example above with 3 x 250W panels wired in parallel, you’d have 41.1 Amps of current from the array. Adding a 25% safety buffer of 10.275 Amps (0.25 x 41.1 Amps = 10.275 Amps) would mean you’d need a PWM controller capable of handling at least 51.375 Amps. Since charge controllers are usually rated in multiples of 10, your best bet would likely be a 60-amp controller.

Xantrex C60 Solar Charge Controller 60 Amps
  • Compatible with 12 and 24 volt battery banks (Charging/dumping voltage setting are user settable)
  • Works with wind turbines, solar panels and hydro generators

Sizing an MPPT Solar Charge Controller

MPPT charge controllers are typically rated for three different parameters: battery bank voltage, solar array voltage, and the current output of the controller. Choosing a controller that’s suited for your battery bank voltage is easy, just confirm that it can handle outputting to a 12V (or 24V or 36V or 48V) battery bank.

Solar array voltage is important because of the MPPT controller’s ability to utilize excess voltage (above the roughly 18V needed to charge a 12V battery) and convert it into extra charging current (amps). This is what allows you to build out larger arrays of panels (wired in series) to increase the voltage of the array’s output. Again, high voltage from the panels, at low current (amps), is an advantage because it improves the efficiency and reduces voltage drop along longer cabling runs.

Lastly, the current output capacity of the controller. This will be a rating (in amps) of the total current the charge controller is capable of pushing into your battery bank to charge it. An MPPT charge controller will actually regulate the current from the solar array, instead of just passing it through like a PWM controller. But, unlike with a PWM, if you exceed the controller’s current rating it will just limit itself to its rated output (though, of course, extended operation this way could reduce the controller’s lifespan). You’ll also be losing out on the charging power of any of the excess current being shed by the controller.

A Victron Energy SmartSolar MPPT charge controller

This Victron Energy SmartSolar MPPT charge controller is a popular choice. It supports up to 150V from the array, and can output up to 70 amps to charge your batteries.

NOTE: There are two schools of thought on sizing MPPT charger current ratings. Some installers recommend “over-paneling” your MPPT charge controller, where the solar panel array is outputting more current than the controller can handle. While this will result in “clipping” of the power to the controller’s max rating during peak times of sun, it means the controller will be outputting maximum current for longer portions of the day, making it easier to plan for the total output from the array.

The other school of thought is to ensure your charge controller is rated above the maximum current output of the solar array, ensuring that it’s never operating at peak, and thus extending it’s lifespan. This is our preferred approach, especially since it can mean you may have enough excess capacity to add more panels, without having to also replace your charge controller.

Using our example of 3 x 250W solar panels, wired in series, from above, we’d need an MPPT charge controller with the following capabilities:

  • Battery Bank Voltage: 12V
  • Solar Array Voltage: 68.4V
  • Controller Current Output: 62.5 Amps (750W panels ÷ 12V = 62.5 Amps), at minimum

The following MPPT Charge Controller from Victron Energy would be a good match (with room for growth). It supports 12V/24V/36V/48V battery voltages, can handle array voltages up to 150V, and can output up to 70 Amps of charging current:

Victron Energy SmartSolar MPPT Tr Solar Charge Controller (Bluetooth) - Charge Controllers for Solar Panels - 150V, 70 amp, 12/24/36/48-Volt
  • Victron Energy SmartSolar MPPT Tr 150-Volt 70 amp: A solar charger gathers energy from your solar panels, and stores it in your batteries, Using the...
  • Victron Energy SmartSolar MPPT charge controller will even recharge a severely depleted battery. It can operate with a battery voltage as low as 0...

Can I Use More Than One Charge Controller?

If the output of your solar array is too large for a single charge controller, yes! There are ways to utilize more than one controller to improve the output and efficiency of your RV’s solar charging system. You can:

  • Split your array in half and run a second pair of wires to a second controller. This could be a good option for a very large array, allowing you to have multiple strings of panels in series, each wired to its own solar charge controller. If you’re planning to use MPPT charge controllers, each can optimize the array voltage for its individual batch of panels (which is especially good if one set is partially shaded)
  • Create a series-parallel array, where you take half of the panels (in series) and put them in parallel with the other half (also in series). This will lower the total voltage while increasing the current output. You’ll just need to be sure that the cabling between the panels and controller is capable of handling the increase in current.

Overwhelmed? Hire Professionals, Instead

While the process of selecting the correct charge controller for your system isn’t all that complicated, it’s certainly not the only thing you may be considering adding to your RV’s electrical system. People adding solar often want to include other components at the same time. Things like:

  • More batteries, to increase storage capacity and help get you through several cloudy days without requiring generator use
  • Upgrading to lithium batteries, to get more storage capacity in the same space, reduce the weight of the same amount of capacity, or just to eliminate the maintenance needed for flooded lead-acid batteries
  • Adding an inverter so you can have 120V AC power when off-grid to run the microwave, charge laptops, or even run the air conditioning

All of these add to the complexity of any solar project. And, even though we’ve done several of our own solar upgrades over the years (including on friend’s RVs), it was a lot of these reasons that we chose to go the professional route with our new travel trailer.

After seeing many of their videos online (and drooling over their incredibly clean installs that are just dripping with “attention-to-detail”) we went with MYT Solar out of Bend, Oregon. Tom & Ely are the husband and wife team behind MYT Solar (and are two of the nicest, most genuine people you’ll ever meet), and they’re well-equipped to help you get the most optimized solar & lithium system for your RV. Want to see for yourself? Check out this video of one of their recent installs:

Stay tuned for our upcoming post and video showing the magic MYT Solar worked on our new trailer: 1,500 watts of solar, 810 Ah of Battle Born lithium batteries, 3,000 watt pure sine Victron hybrid inverter/charger, and 100 amps of DC-to-DC charging from our truck. All on our tiny little 19′ Outdoors RV Creekside 19MKS Titanium. And all BEAUTIFULLY installed!

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Gene Holcomb

Tuesday 21st of May 2024

Thank you for this informative article. We purchased an RV with a nice solar system in place. Now I understand more about the installation.

Interesting to read about your new TT. Since you are getting out of your Newmar are you going to get a home in one of your favorite places?

Thanks again for all your great articles. Have been doing this 35 years, still learning.

TheRVgeeks

Tuesday 21st of May 2024

Great! Glad this was helpful, Gene! We've been out of our Newmar since last spring, enjoying time in our favorite city in North America: Vancouver, BC! But we're out on the road right now in our new travel trailer, putting it through its paces, getting settled in, and (of course) adding in all our favorite gear! 😉 Stay tuned, we'll have lots more to share.

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