Part 1 - Solar Power for your RV

 Many fans of camping in the great outdoors have gone from tent camping with no more need for electricity than what it takes to power a flashlight, to camping in an RV that needs electricity for lights, water, a furnace fan and maybe a laptop and stereo among other things.  If you're camping in a place where you can't plug in to "shore power" and  don't want to use a noise-making generator, this presents a problem, especially if you want to stay out more than a day or two.

190 Watts Tilted into the Sun in the Desert Southwest of the U.S.

My first attempts at switching from campgrounds to boondocking (free camping without any hookups) were highly rewarding in terms of peace, quiet and solitude, but dismal failures in the electrical category.  I had two batteries in my truck camper so was able to go about 5 days before running them down, but that was it.  I wanted to be able to stay out for several weeks at a time or longer, so knew I had to resolve this issue.   Being a huge fan of peace and quiet while camping, a generator was out of the question.

I had a 40-watt solar panel laying around in my garage that came from a small used 5th wheel I had owned years earlier.  The former owner had left it in the rig, but I had never hooked it up.  So I decided to see what it could do.  I didn't have a charge controller, so bought a cheap one online and hooked it all up.  It helped some, but it wasn't enough to keep up with even my minimal power needs.  Again, I was out of power in less than a week.    Worse yet, the cheap on/off charge controller I had bought was damaging my batteries without me even knowing it.

January 2009, with the original single solar panel

Knowing that I never wanted to be dependent on campgrounds again and didn't want to use a generator, I became keenly interested in my electrical system and set out to gain as much practical knowledge about it as I could. 

This nine-page series of articles explains what I learned and what I did to resolve those electrical issues.  And although this deals primarily with RV electrical systems, it can be applied to any 12v off-grid system.  

After reading this you will know how to size your system, choose your solar panels, controller, wires and batteries, as well as how to put it all together.  Links to places to shop for solar supplies are included at the end of the series at the bottom of part 9.

Disclaimer

I am not an electrician, and don't have any professional experience in this field.  My professional background is in computer programming and technical support. So I'm just a guy who had a problem and set out to to solve it on my own.  I like sharing the knowledge I've gained in hopes of helping others, but I don't guarantee your results.  I advise anyone who is not confident or comfortable with doing this kind of work in a safe manner to hire a professional to do it for them.  But if you choose to go that route, it might be a good idea to learn some basics so you can tell the difference between a good and a bad installation.  Finishing this blog should give you the information you need.

RV Solar in a Nutshell

The RV solar system consists of solar panels, wiring, a junction box (if you have more than one panel), a charge controller, batteries, a couple of kill switches and fuses (or circuit breakers).

So you buy some panels, batteries, and a solar charge controller, wiring, connectors, two kill switches and some fuses.  Mount the panels wherever they're going to go.  Now mount the charge controller in your rig, and put the batteries wherever they're going. Wire batteries together.  Then connect the batteries to the charge controller, with a kill switch and a fuse on the positive lead.  

If you have more than one solar panel, connect them together in a junction box with a fuse near the positive terminal of each solar panel.  Next wire the solar panel junction box to the charge controller, adding a kill switch on the positive lead.  If you only have one solar panel, you can skip the junction box and connect the panel directly to the charge controller (but still add the kill switch on the positive lead). 

                               Wiring Diagram for a Typical 12v RV Solar Charging System
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But there are decisions to be made along the way in terms of how big your battery bank needs to be, how many watts of panels you need, what kind of charge controller to use, and the method and size of wiring you want to use.  You should also consider adding a monitor so you can see what's happening with the system.  That is all covered in the rest of this series.

Skip the following section if you are already familiar with basic electricity, watts, amps, volts, batteries, etc., and how it all works in an RV.
 

RV Electricity Basics

Some basic electrical facts to keep in mind when designing your system.

1.  While your RV is plugged in to a campsite's electrical hookup, your 120v appliances run directly off of mains power via the 120v electrical outlet at the campsite.  A converter in your rig takes some of that 120v power and converts it to 12 volt direct current to power your lights, water pump, furnace fan, refrigerator circuit board, etc., as well as to recharge your batteries (older converters weren't powerful enough to recharge batteries, but newer ones are).

2.  When your RV is not plugged in to an electrical hookup or a generator, all your electrical power must come from the house batteries.  This is not a problem for your 12v appliances, but your 120v appliances like 120v phone chargers, laptops, TVs, stereos, etc. won't run on 12 volts.  That's where an inverter comes in.  An inverter takes that 12v dc power and changes it to 120v alternating current (there are 220v models for other countries as well).  Inverters come in many sizes, from quite small to quite large, and not all RVs come with them.  Therefore those who want to boondock and still be able to use 120v appliances need an inverter, assuming you're not going to run them from a generator.  Inverters are widely available from many different sources, from Walmart to automotive stores to countless online sources.  I'm not going to spend any time talking about inverters because they don't really have much to do with solar charging your batteries in an RV.

3.  Understanding what volts, amps and watts are is quite helpful.  A common simplistic and not-entirely-correct-but-good-enough-for-our-purposes analogy is made between electrical systems and plumbing systems.  You can think of an electrical wire as being similar to a water pipe.  The pipe carries water, whereas the wire carries amps*.  Water pressure pushes water through the pipe, voltage pushes amps through the wire.  You can talk about volts and amps together in a unit called a watt.

watts = volts x amps

The above formula is important in solar electric systems because solar panels are described by how many watts they are capable of producing, but your batteries are usually described in terms of volts and amp hours.

An amp is a unit of electricity and is commonly thought of in hourly terms.  For example, if the label on an electric motor says it uses ten amps, that means it uses ten amps each hour.  If my laptop's power supply says the input is 1.5 amps, that means it uses 1.5 amps every hour that it is running. 

Where batteries are concerned, especially deep cycle batteries typically used in RV cabins, there are different sizes and they are described by their physical size as well as how many amp hours of electricity they can store.  

Their physical size is described by a "Group" number.  A Group 27 battery is physically smaller than a Group 31, for example.  This might be important to you if you have limited space in your RV for batteries and need to know what size to get to fit them all in the allotted space.  A list of battery dimensions by group number can be seen on this page at batterystuff.com.  Scroll down the page for deep cycle batteries.

Deep cycle batteries, as mentioned above, are also described by their capacity to hold electricity, in numbers of amp hours.  This is of primary importance to the boondocking RVer.  A 220ah battery can store 220 amp hours of electricity, meaning you could, theoretically, use 10 amps from that battery for 22 hours before it would be completely dead.  It's like a tank that can store 220 gallons of water, from which you could drain 10 gallons per hour for 22 hours before it would be dry.  You would never really want to do this with a lead acid battery though, because draining it completely damages it and shortens its life.  More about that later.

4.  How do you know how full a deep cycle battery is?  Well, lacking a decent built-in meter for tracking such things (and no, the idiot light monitor that comes with most RVs is not decent), you can check wet cells with a hydrometer (best way) or a volt meter.  Hydrometers are available at many automotive and hardware stores for as little as a few bucks.  But if you're using a volt meter, you won't get an accurate reading unless the batteries are at rest, meaning they haven't been charged for several hours, maybe 2, 12, or 48, depending on who you believe, and are not under a load (nothing is connected to them and running).  If they've just been charged they will have a "surface charge" which inflates their actual state of charge (SOC).  It takes time for that to bleed off. And if they are under a load they will read lower than the actual state of charge, depending on how big the load.  So assuming they've been at rest for awhile, they are under zero load (as in not connected to anything), and are at a temperature in the mid to upper 70's fahranheit (25C), these voltages indicate a ballpark estimate on a 12v lead acid battery's state of charge:

12.7v = Full
12.2v = 50%
11.8v = Dead

This can vary by manufacturer and certainly by battery chemistry.  Check with the battery's manufacturer for its own recommendations.

As you can see by these numbers, your 12v battery is, for all practical purposes, dead if it only has 12.0 volts left in it.  That's kind of strange. 

When you charge that battery, you need to apply a voltage that is larger than the current voltage of the battery so the charger's "pressure" is greater than the battery's "pressure" which will force those amps into the battery.  There's much more to it than that, which I don't fully understand, but that's an easy way to think of it.  It sounded strange to me at first when I realized that the recommended charging voltage for my batteries is 14.8 (actually 7.4v for each 6v battery, but for the two of them wired together in series that would have to be doubled - more about this later).  But it needs those extra volts to get all those amps in there in a reasonable amount of time.

So, now what?

With that out of the way, let's continue.  There is a logical progression to this from start to finish.  It goes something like this:

1. Figure out how much electricity you use while boondocking 
2. Use that information to determine how many amp hours you need in your battery bank 
3. Decide which type of batteries you prefer.
4. Decide how many watts of solar panels you need.
5. Decide which charge controller you want.
6. Figure out your wiring needs.
7. Put it all together

So, how much electricity do you use?  That's covered in the next article in the series - Part 2,  Where to Start.

*amps - in reality, the wire carries electrons.  Amps are a unit of measurement of the rate at which those electrons flow through the wire.  But to keep things simple I'll just refer to amps since that's what we're primarily concerned with.