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How To Easily Design And Install RV Solar – Part 1: Planning

PublishedJune 10, 2019

Camp Addict co-founder Kelly just had her four year nomadiversary (she's been full-time RVing since May 2015) and has lived in the same 24-foot travel trailer the entire time. 

Until recently she hasn't had a great solar power setup, despite boondocking all the time.

She's been making do with portable solar panels, which hasn't been the best off-grid power solution for her.

To say that Kelly was hard on her ground deploy solar panels is putting it mildly.

During the 2018 holidays, when we had the opportunity to moochdock somewhere we could easily work on her rig, we finally decided to install rooftop solar. 

It has been working great for her, and no more portable panels have been injured.

Kelly throwing away ground deploy solar panel

Kelly Tossing Ground Deploy Solar Panel

We did not install a whole house inverter.

This is something that a lot of people will install when they are doing a solar installation. Kelly doesn't need one and we were pressed for time.

Eliminating an inverter meant less install time and less educating myself time.

If the need ever arises in the future, we can certainly install an inverter as it is a separate system from the solar side of things. 

Many people just opt to do them at the same time because it makes sense. If you are going to be upgrading your RV's electrical system, you might as well go all in.

Kelly's solar panel installation is pretty simple compared to some RVs. She doesn't have an onboard generator (common with motorhomes) so we didn't have to consider this. 

And, as stated directly above, there is no whole house inverter to deal with.

  • This is a very technical post and can get ultra-confusing fast. I do my best to simplify the subject matter, but this stuff is confusing, especially if electricity isn't your strong suit (I'm pointing a finger at me)! Hopefully, your head isn't spinning too much by the end of this.
  • This is part one of Kelly's solar installation where I talk about general planning concepts (my thought process, etc). In part 2 I cover the steps we took to install the solar system, including the parts and supplies we used.

Who Do You Believe?

There are many different approaches and schools of thought when it comes to planning out a solar panel installation on an RV.

It can be a bit daunting if you've never done a full-blown solar install before. Especially if electricity isn't your thing.

I've never done a rooftop RV solar installation before. Back in the day when I was going through aircraft mechanic school, electricity was my worst subject. 

So yeah, I seriously dragged my feet when it came to studying up on how to tackle this project.

Once I worked up the courage to start the planning process, I did what most people would do. I started looking online.

OMG! What a rabbit hole that turned into. As I said, there are many different schools of thought on how to install solar.

And I'm sure some of them are even correct. But, dang, it gets confusing fast.

My Experience Only

I'm not going into great detail regarding how to design YOUR solar system.

I'm just writing about the steps I took to plan Kelly's setup. My thought process, if you will.

There are SO MANY variables when it comes to a solar system installation that it's impossible for one post to cover every possible situation.

Yes, you may learn something (I hope you do!), but your rig is different than Kelly's so your setup would be different.

Installing a solar system isn't easy (when done right). It takes a lot of planning and some mechanical and electrical know-how.

If you don't know what you are doing, find someone who does (good luck with that!). Otherwise, I hope this post helps get you started. Look over the resources yourself, and have at it!

Resources I Used

To stem the madness of researching good solar panel resources so that I could get to the business of planning, I quickly settled on one main resource.

Of course, when necessary, I did do some poking around online.

My point is that I didn't spend a huge amount of time trying to find valid sources on how to plan an RV solar panel installation.

Trust me, you can waste many, many, many hours trying to figure out what is good online information and what is bad.

(Spoiler alert - most of the stuff you read online is crap. #shocker)

Basic Solar Installation Information

The main resource I used for RV solar install information is Jack Mayer's site.

This is the 'Jack' that I will be extensively referring to in this post.

I first learned about Jack many moons ago when I was early in my RV living planning/fantasy stage.

He and his wife had a brand of high-quality fifth wheel that I was lusting over, so I stumbled across their site way back when.

I've also seen Jack referenced many places online.

In other words, he's been around and I had confidence that his information was good.

There are three specific sections/pages of Jack's site that I spent a great deal of time on:

Jack's site certainly isn't pretty by today's standards (I'm guessing it's the same style that he's been using since starting it in the early 2000's).

Which usually means I'd quickly close the tab and move on.

But because it's chocked full of such good info, I persevered and learned a lot.

(Though I do find it incredibly hard to read - there's a reason modern websites don't look like this anymore). 

So bear with the site design and get to learning!

As of this writing, Jack's solar pages were last updated in 2017.

Definitions Of Terms Used

Throughout this post, you will find some abbreviations that you may not be familiar with.

Below are definitions of some abbreviations used.

  • Amps - Unit of measurement for electrical current. Short for Ampere.
  • AWG - (American Wire Gauge) A standard for wire conductor size predominantly used in North America. The smaller the AWG number, the larger the wire size diameter.
  • Combiner Box - A junction box that allows you to 'combine' all the leads from your solar panels into one pair of leads (positive and negative wires) that then run to your solar controller. This greatly reduces the number of wires running through your RV.
  • Home Run - The wires running from your solar panels to the combiner box. These will be the wires on your roof and should be UV and weather resistant.
  • Imp - Solar panel current at maximum power. The maximum number of amps a solar panel can produce under perfect conditions. Use for figuring out how many amps a solar panel array can produce (depending on if using series or parallel wiring, or a combination of both), thus how large of a solar controller you need.
  • Isc - Solar panel short circuit current, or how many amps a panel will produce if the positive and negative wires of a panel are connected. Isc is used when figuring out circuit protection requirements (fuse for individual panels).
  • Solar Array - A fancy way to say all the solar panels in your system.
  • String - A string of solar panels is two, or more, panels wired together in series. You can have multiple strings of panels (panels wired in series) in a solar array. These strings of panels will be wired together in parallel at the combiner box.
  • Vmp - Solar panel voltage at maximum power, or what the panel voltage will be when producing the Imp rated power. Use for figuring out wire sizing from panels to controller. Total system Vmp will vary depending on if you are wiring your solar panels in series or parallel (or a combination of both).
  • Voc - Solar panel open circuit voltage, or the voltage the panel is capable of producing when the sun is hitting it but no wires are connected to it. Only really used to calculate solar controller size.

Wire Size Calculator

One of the concepts that Jack hammers home is wire size. 

The whole point of having solar panels on the roof of your RV is so that your house batteries are recharged.

They also create power to use throughout the day while the sun is out.

Wiring transfers the power generated from the solar panels, down into your RV, to the solar controller, and on to the batteries.

Depending on the length of your RV, and where the solar components are laid out, you could have some long wiring runs.

The more time electricity spends traveling down wiring (due to long wiring runs between solar components and the batteries), the more voltage loss there is.

In other words, you are losing some of the power the solar panels are generating due to the wiring the power is having to travel thru.

You mitigate this loss by using larger sized (gauge) wiring.

But you don't want to use too large of wiring size for a few reasons.

Only Use The Size (Gauge) Wire You Need Because:

  • The larger the size (gauge) wire you use, the more expensive it is per foot (copper is used to make quality wiring - the type you will be using - and copper isn't cheap)
  • The larger the size (gauge) wire you use, the more difficult it is to install (thicker wires take up more space and is harder to 'shove' through the holes it has to run thru)
  • Larger diameter (gauge) wire is heavier, and weight is a consideration for most RVs

Throughout the planning process, I used this calculator to give me an idea of what size wire I needed for each portion of the solar installation:

Voltage Drop Calculator

To properly use the above voltage drop calculator you will need to know the following:

Laying Out An RV Solar System

Before you can start spending your hard-earned money on solar components, you need to know where things will go.

This will give you an idea of how many solar panels will fit on the roof of your RV, and how much wire you will need to purchase to hook everything up.

Mapping The RV's Roof

The first step I did when it came time to design Kelly's solar system was to map out the roof of her trailer.

I needed to see how much space (and what shape the space was) we had to work with.

We climbed up on her roof, armed with a long tape measure and a pad of paper. 

I then sketched the rough location of whatever was on her roof that would get in the way of the solar panels.

(Air conditioner, roof vents, TV antenna, etc.)

We measured the distance (and size) of each item from the edges and the front and rear of the RV.

The usable area the roof was also measured so we'd know how much total area the roof had.

Roof ready for solar panels

Using Cardboard To 'Play' With Solar Panel Layout

Once back on the ground, I transferred these measurements to a clean piece of paper.

Graph paper with small squares works well for this.

The idea is to decide that one square on the graph paper represents 'X' number of inches in the real world.

Then draw out the rectangular shape of the roof, to scale, as a starting point.

Add all the existing items on the roof (again, to scale) at the proper places, per your measurements.

You'll end up with a scale drawing of your roof, with all the obstacles that you have to avoid during solar panel placement.

I then cut out scale mockups of the solar panels I was considering using to figure out placement.

You need to take into consideration shading of the solar panels by taller objects on your roof, such as the air conditioner.

How Many Solar Panels And Where

We had an idea of how many watts of solar we wanted to install on Kelly's roof.

No, I didn't do any fancy calculations taking into consideration how much power she consumes in a day, blah, blah, blah.

Rather, I relied on the experience of using ground deploy solar panels for several years.

This gave me a very good idea of how much power she needed. 

Then I added 50%, because solar panels are relatively cheap, she had the room on her roof, and not every day is perfectly sunny.

When there are clouds in the sky, making for a bad solar day, you are gonna wish you had huge amounts of solar panels on your roof.

Otherwise, you are going to have to pull out the portable generator, and that's a pain.

Rooftop solar install done looking aft

600 Watts Of Rooftop Solar

We settled on 600 watts of rooftop solar, which meant three 200 watt panels.

This included two rectangular shaped panels and a single square panel.

It was the best fit for her roof.

Of course, as is the case with most plans, things went out the window when install time came.

We removed her TV antenna that she never used.

It took up a huge amount of space.

This allowed us to install the panels a bit differently than originally planned.

Be prepared to be very flexible (literally and figuratively) as you do your solar install.

Things will come up that you didn't anticipate and you will always end up having to make more runs to the hardware store (or make additional Amazon orders) than expected.

Getting Wires From The Roof Into the RV

I decided to run the wires from the roof into the rig via the rooftop refrigerator vent.

This eliminated the need to punch another hole in the RV's roof.

It also happened to guide the wires right where I wanted to send them into the rig.

I opted to use an MC4 'Y' connector to combine the two rectangular solar panel's wires into a single set of wires to run across the roof and down the fridge vent.

This eliminated extra wires both on the roof and down the vent.

MC4 branch connector and inline fuse

MC4 Branch Connector Combines 4 Wires Into 2

This meant four wires (two wires from the rectangular solar panels and two wires from the square panel) went thru the roof vent as I opted to mount the combiner box right inside the rig.

Where To Put The Solar Controller?

We discuss solar controller location in more detail below, but here is a quick summary:

You generally want to put the solar controller as close to the RV's batteries as you can. Kelly's RV batteries are on the tongue, (Kelly now has lithium batteries) as is the case with many travel trailers.

We installed the solar controller in the forward baggage compartment, which is close to the batteries while being protected from the weather.

Kelly Now Has Lithium Batteries!

In July 2019, about 6 months after we did her solar install, she upgraded to two lithium batteries.

Instead of mounting these on the tongue where her 6-volt golf cart batteries were, we installed them inside the baggage compartment, right next to the solar controller.

Putting the lithium batteries here served two purposes:

  1. They are out of the elements and secure from theft.
  2. They were as close to the solar controller as possible. We could not have installed them any closer.

Where To Run The Wires?

The general idea was to get the wires from the solar panels on the roof, through the roof, into some sort of a combiner, down under the belly of the trailer (belly isn't enclosed), forward to the solar controller (which means back up thru the belly), down from the solar controller thru the belly again, forward to the batteries.

Whew!

Wiring installed under belly

Wiring Running Below Floor

That was a mouthful!

Sounds complicated, but fortunately on Kelly's rig, it was a fairly easy process.

As easy as it could be.

Which means it was still a pain!

Once the wires were through the roof, I wanted to run them straight down the pantry and through the floor. 

Fortunately, there was already a hole in the floor that had enough space for me to run two more wires.

Then from there, I would run the wires forward, zip-tying them to the frame as I went.

Battery Monitor

I consider a battery monitor a mandatory piece of equipment with any RV solar installation.

Why?

Simply because without a battery monitor you cannot know for sure if your batteries are being charged completely each day by your solar system.

Furthermore, you have no idea how much power you use on a typical day.

If you're on the fence about it, just trust me.

A battery monitor is a key piece of equipment for any RV solar installation.

Victron BMV-712 Battery Monitor display

Victron BMV-712 Battery Monitor

What does a battery monitor do?

It simply monitors the flow of power in and out of the battery.

It keeps track of how many amp-hours of power leave the battery, and how many are put back in.

It also does things like monitor battery voltage and temperature (if you have an optional temperature sensor).

It can relay this information back to the solar charge controller via Bluetooth (in the case of the Victron system we installed).

When installing a battery monitor, make sure there is no load between the shunt and the battery.

If there is a load between the shunt and the battery, the shunt won't be able to 'read' it and the battery monitor will not 'record' this load.

You may need to move some ground wires around and install a distribution block to allow for this. We had to do this with Kelly's solar install.

General Wiring Concepts

Wiring is the most critical component of your solar installation.

As explained above, the power that your solar panels will be generating is 'transported' from the panels, to the solar controller, and then the RV's house batteries via the wiring.

What Size Wire To Use?

If you use too small of a gauge (diameter) of wire, or use cheap wire that doesn't use copper strands, you will inhibit the 'flow' of power. 

You'll have a solar system that will not be performing as expected and you will be a sad panda.

Jack is a believer in using a gauge of wire that will give you no more than a 2% voltage drop (preferably a 1% drop) throughout your solar setup.

You calculate voltage drop using a wire size calculator.

In order to achieve little voltage drop, you may end up needing large gauge wire.

It depends on both the length of a wire run and the amperage your solar system is able to produce.

There are several issues with using large gauge wire, so you may or may not want to follow Jack's advice as far as voltage drop.

If you look online, you will get people saying all sorts of percentages of voltage drops are OK, but you definitely want to stick in the low single digits.

I was happy using wiring that gave a 2-3% voltage drop on some runs, and a lower voltage drop on other runs.

But that's just me.

Some people may be in the Jack camp and won't be happy unless you have a 1% voltage drop.

Jack's Thoughts On Wire Size

As stated above, Jack is a believer in using large enough wire to have no more than a 2% voltage drop in each wiring run.

He prefers a 1% voltage drop. 

This potentially means using a rather large wire size (diameter).

Jack likes to use 4 AWG wire from the solar panel wire combiner to the solar controller.

Many solar controllers will only accept a 6 AWG wire size max (which is a smaller diameter than 4 AWG).

He says it's OK to trim the diameter of a 4 AWG wire to fit into the 6 AWG 'hole' in the solar controller.

From the solar controller to the batteries, Jack likes to use 4 AWG here as well (or even 2 AWG wire).

Again, he says it's OK to trim the diameter to allow it to fit into the solar controller output terminals.

Keep in mind that this may be overkill for your particular setup.

Use the voltage drop calculator to figure out what your RV requires.

This size wire is probably only required if you have a large solar array or a long RV (requiring long wiring runs).

Do Calculations Based Upon Max Capacity

You want to do your wire size calculations based on the maximum capacity of your solar controller.

This is true even if you aren't using the controller's full capacity right now.

You will want to allow for possible future system expansion (unless you don't have any more room on your roof for solar panels).

Keep in mind that you may want to add ground deploy (portable panels) capability in the future if you aren't already.

portable solar panels at the Tetons

Ground Deploy Solar Panels

Run the ground deploys through your solar controller so you don't end up with two controllers working against each other.

Don't forget to factor in ground deploy panel wattage into your wire size calculations. 

And factor in the extra power input from any ground deploys when you are sizing your solar controller.

So. Much. To. Consider.

Wire Size We Used

Keep in mind that Kelly's rig is fairly small.

24 feet with 'only' 600 watts of solar on the roof (and no plans to ever expand).

If you have a longer RV (which, most likely, will result in longer wiring runs) and/or are installing more solar, then you will have to use the appropriately sized wiring for your situation.

This is the wiring size used on Kelly's solar installation:

  • 10 AWG from the solar panels to the combiner
  • 6 AWG from the combiner to the solar controller
  • 6 AWG from solar controller to batteries

We used welding cable (high-quality copper, very flexible) for the 6 AWG runs and 'solar panel' cable (UV, weather, and corrosion resistant) for the 10 AWG runs on the roof.

Use Quality Wires

For wiring between the combiner box and the solar controller, and the controller and batteries, you will want to use welding cable.

Welding cable comes in the larger diameter that you'll need.

It's highly flexible, and is made with pure copper strands.

For the wiring from each solar panel to the combiner box, you will want to use a UV resistant cable (Jack recommends using what is called 'tray cable').

This wiring will be running along the roof of your rig.

Therefore it needs to be able to withstand the sun's UV rays.

Disconnect Switches

You need a way to disconnect the solar panels electrically from the solar controller, and to be able to disconnect the solar controller electrically from the batteries.

We did this with surface-mount circuit breakers.

Solar controller breakers closeup

Circuit Breaker 'Disconnect Switches'

We used a 30-amp breaker between the solar panels and the controller (because the solar panels could, theoretically, produce no more than a little under 30 amps). 

And we installed a 40 amp circuit breaker between the solar controller and the batteries (the solar controller we used can output 30 amps maximum, so we went with the next size up).

MC4 connector and inline fuse

In-Line MC4 Fuse Holder

We also put a 15-amp fuse (via an MC4 in-line fuse holder) at the 'Y' connector that combined the two rectangular solar panel wires and at the square panel (for a total of two fuses). 

This isolated the panels in case they had shorts.

Catastrophic Fuse

You need to have a catastrophic fuse right at, or very close to the battery bank.

This protects the wiring in case there is a direct short to ground on the positive wiring from the battery.

If you don't have a catastrophic fuse, and a positive lead anywhere between the battery and your power distribution panel goes to ground, things will start to go sideways quickly.

An RV battery bank is capable of producing some serious amperage output in a very short amount of time.

There will be some arcing and sparking (to say the least).

The catastrophic fuse is there to prevent this from happening.

If a major positive cable somehow gets grounded, the fuse will 'pop'.

Because it is right at the battery (in other words, it will 'carry' the entire max load of the RV), this fuse is rather large.

Catastrophic fuse installed

Catastrophic Fuse

We used a 50-amp fuse which is plenty for Kelly's rig (no inverter). Your RV may require a larger fuse.

Your RV will come with a catastrophic fuse already, and it may be sufficient (depending on its capacity, location, etc).

Kelly's rig had the fuse mounted in a horrible, very exposed location, and was pretty junky quality. 

No shocker here that an RV manufacturer didn't spring for a quality component.

A couple of years ago the original one corroded and caused her entire electrical system to not work, so we replaced it.

But we replaced it with the same style, at the same location.

I wasn't happy about this, so since we had to move some wiring around for her solar installation, it was time to install a better catastrophic fuse.

Use Quality Hardware And Properly Attach

You are going to be having to make wiring leads as you do your installation.

It is next to impossible to purchase pre-made cables in the exact length you need, in the quantity you will want.

Plus, it may not be possible to run a pre-made cable (with big wiring lugs on the ends) through some spots in your RV as you snake the wiring thru your rig.

This means you will be getting familiar with properly installing wiring lugs (end connectors).

Use quality wiring lugs and attach them properly.

Jack recommends using mechanically connected wire lugs.

Wire lug stamped

Mechanically Connected Wire Lug

If you are going to use solder lugs (Jack recommends against using these), use fusion lugs for the larger sized wiring that come with solder and flux in the barrel already.

You are going to want to use an anti-oxidant compound when assembling the wire lugs onto the wire.

Apply to the wire end (rub it in) and put a little into the lug itself before crimping.

Lastly, use high-quality adhesive heat shrink that is color-coded (red for positive and black for negative wires).

The adhesive that it inside the heat shrink will 'ooze' out and create a corrosion-resistant barrier.

Combiner Box

Each solar panel has two wires (a positive and a negative).

Those wires have to get from the roof down into your rig, eventually making their way to the solar controller.

Instead of running 6 wires (3 solar panels for Kelly's installation, 2 wires per panel) from the roof, all the way to the solar controller (which we opted to install at the front of the rig), the idea is to 'combine' the solar panel wiring together into one pair of larger gauge wire.

This means you have fewer wires to run a (potentially) long distance.

You use some sort of a combiner that allows you to consolidate the 6 wires (in Kelly's case) into 2 wires.

The wires running into the combiner (from the solar panels) are smaller gauge wires, and the two wires running out of the combiner are larger gauge wires.

Types Of Combiner Boxes

Solar combiner boxes are typically mounted on the roof, in a central location, as to limit the wire lengths (runs) from all the solar panels to it.

Then there is a hole drilled in the roof so that the wires can run from the combiner box down into your rig, and eventually to the solar controller.

You can either purchase a pre-made combiner box, or you can make your own using power distribution blocks placed inside a weatherproof box.

If you make your own, you have to use the proper method to weatherproof the holes drilled into the box that allow wires to pass through.

For smaller rigs (that don't require long wire runs from the solar panels to the combiner), you may be able to put the combiner inside.

This eliminates the need to have a weatherproof combiner, though you still need a weatherproof method to get the solar panel wires inside.

Combiner box installed covers on

Combiner 'Box' Inside RV

With Kelly's installation, we opted to combine the wires inside her RV, right by where they came in through the refrigerator vent. 

This inside location protects the combiner from the elements and is conveniently located inside the pantry of her rig.

Many (most?) rooftop solar installs will use a water/weatherproof combiner box on the roof itself.

Kelly's install is a pretty unique situation.

 Not only was the refrigerator roof vent conveniently located, but there was an ideal location to put a combiner just inside the roof vent (in a pantry).

Solar Charge Controller

The solar controller is the heart of any solar installation.

It converts the energy produced by the rooftop (and ground deploy, if you are using them) solar panels into something that is usable to charge your RV's batteries (and to supply 12-volt electrical power used during the day).

The solar controller was the single most expensive item for Kelly's solar installation.

Keep in mind that we did not install a whole house inverter.

That tends to be a pricey bit of equipment as well.

What Size Controller?

You want to 'size' your solar controller based on the Imp rating (amps) of the solar panels you use.

Remember to factor in any ground deploy panels that you may be using (currently, or in the future).

Voc rating (voltage) will also factor into your charge controller selection, as a controller will have a max input voltage it can handle.

Whether you wire your panels in series or parallel will determine the actual Voc number you will use when determining charge controller sizing.

Your solar controller will want to be able to handle the maximum amount of amps that your solar panels (rooftop and ground deploys) will be able to produce.

Again, this is the Imp rating of your panels, the total of which will be calculated based upon if you are wiring your solar panels in series, parallel, or a combination of both.

Keep in mind that you probably will never see this max Imp output of your solar panels as it is based upon a perfect scenario.

The perfect scenario would be the perfect temperature, the perfect sun angle, panels being perfectly clean, etc, etc, etc.

Victron SmartSolar MPPT 100:30 solar controller

Victron SmartSolar MPPT 100/30 Solar Controller

This means that if you are on the fence with the size of the controller you need (calculations indicating that you could jump one size up), you may be OK with the smaller size.

Sometimes there is a significant price difference going to the next sized solar controller, but it's also smart to consider the future.

If you want to install more solar down the line, then you may just eventually need the larger solar controller anyhow.

But if you are on the fence, consider that some solar controllers are able to 'accept' more than their rated amperage input.

They just can't 'do anything' with those extra amps - they are essentially wasted.

Finally (we've touched on this before), keep in mind any future possible expansion of your solar setup.

If you have more room for solar panels on your roof, you might want to initially go with a solar controller capable of handling additional panels.

You may want to do a theoretical max solar input calculation based upon how many panels could fit on your roof (assuming it's not some crazy wattage that you would never end up putting up there), and initially purchase a solar controller capable of handling this much power.

Solar Controller Location

If you use an MPPT solar controller (and you should if you are installing any sort of decently sized solar array), then the controller is going to be able to output more amps than it takes in (due to being able to convert the higher voltage input into extra amperage output - not going into detail about that here as it will be the subject of a future Camp Addict page).

This means that you are going to want the shortest possible wiring run from the solar controller to the battery bank.

This will minimize the size of wire needed and voltage drop (energy loss).

Solar controller sub assembly installed

Solar Controller Mounted Inside Forward Cargo Pass Thru

If possible, locate your solar controller right next to the batteries.

But this isn't always possible, especially if your batteries are hanging out in the open on the tongue of your trailer, like Kelly's are (Kelly now has lithium installed right next to the solar controller).

We ended up installing the solar controller inside the cargo pass-thru compartment located at the front of Kelly's rig.

This gets the controller pretty darn close to the batteries (minimizing wire run), while being protected from the weather.

Solar Controller Sensors

The MPPT solar controller that you (most likely) will be installing is a 'smart' controller.

This means it's able to adjust how long it's charging in each of its charge modes based upon battery condition (voltage) at the beginning of the day.

It can also show parameters such as battery temperature and voltage during charge.

To know what is up with the batteries, a smart solar controller relies on sensors.

This includes voltage and temperature sensors.

Of course, the solar controller you buy might not come standard with said sensors.

We installed a Victron solar controller and a Victron battery monitor. 

The battery monitor can sense battery voltage and temperature.

The battery monitor 'talks' to the solar controller via a Bluetooth network.

This way the solar controller knows exactly what is going on with the battery.

This allows the controller to compensate for any voltage drop between it and the battery, as well as to know if the battery is hot and cut back on the charging if so.

Solar Panel Selection

Solar panels come in all different shapes, sizes, styles, power outputs, etc, etc, etc.

I'm not going into a huge explanation of solar panels - that's for another time.

Rather, I'm just going to hit the basics about what you should consider when shopping for solar panels and what to think of when wiring them together.

Hold on tight - things may get confusing here.

The first rule of thumb is that you will want to match all of the solar panels in your system (including ground deploys, if you are using them) as closely as possible when it comes to Vmp and Imp ratings.

Of course, this will depend on if you are wiring your panels in series, parallel, or a combination of both.

You can use different wattage panels in your solar array, but there are a few things to consider:

  • If you are wiring two+ panels in series (to increase the overall voltage), you will want to match the wattage of these panels (or more specifically, the Imp rating). Panels wired in series will have their individual amperage (Imp rating) reduced to the lowest Imp of the string. (More on this later.)
  • If you aren't wiring panels in series, then you can mix and match solar panel wattage rating. They do need to have the same (or very close) Vmp ratings (more on this in the series/parallel wiring section).

Solar Panel Voltage

Ideally, you want to use a higher voltage panel (28+ volts Vmp rating) in your system.

However, higher voltage panels tend to be rather large, and you may not have room on your roof for these (we didn't on Kelly's rig).

An alternative is to use lower voltage panels and wire them in series to achieve this higher voltage.

This will require you to have an even number of panels (we installed 3 on Kelly's roof so we could not do this) - more on this in the series/parallel wiring section below.

MPPT solar controllers perform best when they receive a voltage input of between 2 and 2.5 times nominal battery voltage (for a 12-volt system, nominal voltage is 14 volts).

Too much voltage isn't efficient, so hit the sweet spot if possible.

For a 12-volt system, the ideal Vmp rating would be in the 28-35 volt range (again, this can be achieved by wiring panels in series if possible).

Series Or Parallel Solar Panel Wiring

When you have more than one solar panel in your solar array, you have to decide how to wire them together.

Either in parallel or series, or a combination of both.

There are pros and cons to each method.

  • Don't know the difference between series and parallel wired circuits? Here is the Wikipedia article that explains this, so I don't have to.

When wiring solar panels in parallel, the amperage (Imp) rating will combine, but voltage (Vmp) won't.

Voltage (Vmp) will be limited to the lowest Vmp rating of an individual panel (thus why it's important to match Vmp as close as possible).

When wiring solar panels in series, the voltage (Vmp) will combine, but the amperage (Imp) won't.

Amperage (Imp) will be limited to the lowest Imp rating of an individual panel (so don't be wiring a 160-watt panel in series with a 240-watt panel, as the amperage will be reduced to the 160-watt panel, thus 'wasting' the extra watts/amps on the 240-watt panel).

Wiring solar panels in series will increase the voltage of that string, without increasing the amperage.

This allows you to use smaller gauge wire for the home run, or make the home run longer without having to increase the wire size.

As explained above, MPPT controllers like higher voltage inputs, so this could be a good thing.

Solar Panel Shading

One thing you need to consider when figuring out where your solar panels should be placed on your RV's roof is shading.

Look at other items on your roof (air conditioner, TV antenna, roof vents, etc.) and think about how they will cast shadows on your solar panels.

Shadows are a bad thing as even a small shade spot will greatly reduce power output.

Not only can things on the roof shade a solar panel but so can trees. 

Don't park under the shade of trees and expect your RV's solar system to produce a lot of power.

It just doesn't happen.

How you wire your solar panels (series or parallel) will determine how shading affects the entire system. 

Panels wired in series will all be affected if only one of the panels is shaded.

Panels wired in parallel will only result in the panel being shaded having reduced output.

So it's all a trade-off.

Wiring your panels in series will increase the total panel voltage, which can be a good thing with an MPPT controller, but you have more issues with panel shading affecting solar output. 

Wiring panels in parallel means fewer issues with shading affecting the overall power output but doesn't let you increase total panel voltage. (See parallel rules below.)

If you have an even number of panels (say 4 or 6), then you can use a combination of series and parallel wiring to take advantage of the increased voltage, yet limit the shading effects.

Remember, when wiring solar panels in series, you need to match the amperage (Imp) rating (which means use the same wattage panels in a series string).

The Imp rating of a string will be the lowest Imp rating for a single panel in the string.

You can use a combination of series and parallel wired panels in a solar array, but 'parallel rules' apply at the combiner box, as each string of panels will be wired in parallel at the combiner.

Parallel Rules:

  • Current (Imp) of each string will be added together
  • Voltage (Vmp) of combined strings will be limited to the lowest Vmp of a string
  • Restating the above point: Make sure the voltage (Vmp) of each string are the same as one string with a lower Vmp will 'bring down' all other strings (Vmp will be reduced to lowest Vmp of all strings)

I don't know about you, but my head is about to explode after writing this section on solar panels.

Yes, it gets confusing.

At least it does for me.

Miscellaneous Solar Panel Installation Notes

Here are some random points about mounting solar panels to the roof of your RV:

  • If you have a fiberglass roof, use 3M VHB tape or 3M Fast Cure 5200 Marine Adhesive to help secure the solar panel mounts to the roof.
  • Use stainless 1 -inch #10 or #12 screws (embedded in a caulk puddle) to secure solar panel mounts to roof. Use the appropriate type of caulk for your roof material. You only need to use one screw per leg if your roof can use tape or adhesive (it's a fiberglass roof), otherwise use two screws. Aim for roof trusses/studs so that the screws have something to 'bite' into. 
  • Attach solar panel home run wiring to the roof with puddles of caulk (use proper caulk type suitable for your roof material) every 3 to 5 feet. When dry, over puddle with more caulk.
  • It's OK to use MC4 connectors ('Y' branch connectors) to combine solar panel wires to make a home run (wired in either series or parallel depending on how you do things).

Conclusion

Hopefully, this post gives you an idea of what is involved in planning a quality RV solar installation.

There is a lot to wrap your mind around, and it is a (potentially) confusing subject.

Remember that the above information is based upon general solar install ideas, with specifics being, well, specific to Kelly's RV (your RV will be different).

Also keep in mind that this solar installation was a very simple one as she doesn't have a whole house inverter.

Nor does she have an onboard generator that needs to be factored in (wiring of things get more complicated when you have an onboard generator).

If you don't posess the technical chops to do the install/planning yourself, find someone trustworthy that does.

If you want to tackle the job yourself, become very familiar with the resources I used.

I learned a ton from Jack, and you will too!

Always plan for a maximum solar scenario.

You might start with a smaller solar array, but you may want to expand in the future.

For example, if you initially install 200 watts of solar on your roof, and size all the wires and the solar controller to only be able to handle 200 watts of power, you are screwed if you want to put more solar on your roof.

Rooftop solar install done looking forward

600 Watts With Very Little Room For Expansion

If you wanted to leap to 600 watts, you need to run new wires and replace your solar controller. 

Trust me, you only want to run wires once.

Give yourself some expansion room.

The only reason why you wouldn't want to do this is if you are already installing a monster solar array, or only have room on the roof for what you are installing (keeping any portable ground deploy panels in mind as well).

Bottom line is that a quality solar installation is well worth it.

Especially if you spend a lot of time camping off-grid as we do.

Kelly is extremely happy with how her solar setup has been performing (as am I).

All the trouble and aggravation that I experienced during the install ultimately was worth it in the end.

Read part two to learn the steps we took to install a 600 watt RV solar system on Kelly's RV.

Marshall Headshot

Camp Addict co-founder Marshall Wendler brings his technical expertise to help explain RV products in an easy to understand fashion. Full-time RVing since April 2014, Marshall loves sharing his knowledge of the RV lifestyle.

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  • I’m basing my install on using 125 amp hours a day..not sure how much battery storage I should use..my research says 250 ah worth of batteries in paralell…comments?

    • Hey Rick,

      The topic of batteries and battery capacity is beyond the scope of this article. But what the heck, I’ll try and give you my thoughts.

      When you say ‘batteries in parallel’ that implies you are thinking of using 12-volt batteries. If you used 6-volt batteries (lead acid), you would wire them in series if you had two, and in a combination of series and parallel if you were using four.

      Of course, to my knowledge, only lead acid batteries come in 6-volts. This is a moot point if you are using lithiums as they would be wired in parallel.

      250ah of battery capacity… Are we talking lead acid or lithium batteries? If lead acid, then any battery bank that had a 250ah capacity would only have 125ah usable, as lead acids should only be drained to 50% capacity. This probably isn’t enough battery capacity if you are using 125ah a day. Won’t leave you any overhead for crappy solar days (bad weather).

      Lithium batteries are able to use MUCH more of their rated capacity, so you’d get a lot closer to having 250ah of usable with a 250ah lithium battery bank.

      Battery capacity is a balancing act. What happens if you are boondocking and have a stretch of bad weather which results in a string of bad solar days? Do you have enough battery capacity to hold you thru, or will you need a portable generator for these bad weather days?

      Even with a typical bad solar day (bad weather) you are going to have some sort of solar incoming. And the larger the solar array, the more it will be able pull in some energy on bad weather days.

      So, assuming you are talking 250ah of lithium batteries, you should have 2 days of normal electrical use without any solar input. Of course you will have some solar input, even on bad weather days, so you can stretch that out to more than 2 days.

      But is 250ah enough for a week of crappy days, even with some solar input? You could always do what you can to cut back on energy usage in that case. If you don’t want to carry around a generator.

      And how much money do you have to purchase lithium batteries? They aren’t exactly cheap. While it would be nice to have four lithiums, that’s a chunk of change.

      To my knowledge (and I haven’t really researched them), lithium batteries come in roughly 100 amp hour capacities, per battery. So you’d need 3 batteries at least. So roughly $3k at today’s battery costs. Not an insignificant amount of money, unless you full-time RV and boondock all the time. Then they will pay for themselves relatively quickly.

      I also have no clue if 125ah is a realistic number for you. I don’t know what rig you have (or are thinking of getting). What you plan to power. Etc, etc, etc. Everyone’s electrical needs are highly personalized. I don’t use much, whereas I have friends that use many multiples of what I do because they like their electrical devices and don’t want to live minimally.

      Hope I didn’t muddy the waters with that answer. It’s going to be a lot of trial and error to figure out what works for you. Just plan for system expansion and start out small. That’s the real takeaway here.

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