Tuesday, February 24, 2015

Considerations for Installing Solar on your Airstream

There have recently been some great discussions in the comments of a couple of my Instagram posts describing our experiences with our solar panels. Enough people have asked about our system and how it’s worked for us that I thought I’d take some time to write up a blog post with some of my latest thoughts after about a year of using our setup. Of course, I’ve already written about some of this in “Planning and Layout of the Solar,” but I’ll try to make it a little more clear here and add some thoughts on what I might do differently if I did it again and how others might get started in phases with a solar installation or decide if solar is right for them at all.


The first thing you really need to know is your power usage while using your trailer. Each light, each fan, your water pump, the blower on your furnace, the radio, the 12V outlet charging your cell phone, even the electronic controls for your fridge and that little blinking propane detector pull some amount of energy from your batteries.

Here are a few numbers I’ve recorded:

Radio 0.4A
iPhone charger 0.5A
LED lights (3 @ 9 diode bulbs) 
Note: You can figure 0.1A per fixture so only 0.1A for a reading light
Incandescent closet light (still haven’t replaced) 1.3A
Bathroom fan 1.3A
Fantastic Fan (Level 1/2/3) 1A/1.5A/1.8A
Water pump 5A
Furnace blower 5A
Inverter turned on w/o anything plugged in 1.3A
Laptop plugged in and charging 3.5A
Laptop plugged in and running 5.7A

Your usage is calculated in “Amp Hours,” the the equivalent of using 1 amp of power for one hour. Based on my numbers above, charge your iPhone for 2 hours at 0.5A, that’s 1 amp hour. Flick on the main overhead lights (our trailer has 8 in the main room), that’s just 1 amp hour for each hour you run them. Note, if you haven’t upgraded to LED’s yet, do not read further. Pick up the phone and call Dan at LED4RV now, and he’ll talk you through what you need. This is the single easiest and most cost effective thing you can do to get started managing your power. Without that call, those overhead lights are going to cost you over 10 amp hours each hour and without question, you’ll be waking up to dead batteries.

While it might be hard for you to figure out what your usage might be, as a starting point, our experiences closely monitoring our power usage in our Airstream have shown that we use an average of 2-2.5 amps of power each hour. Of course, that’s not consistent; We might use 4 amps while we’re heating up the trailer in the morning and listening to the radio while making breakfast, only 1 amp while intermittently running the Fantastic Fan during the day, 5 amps while making dinner and listening to music with the lights blaring and heat on and 3 amps with the heat turned down and us all snuggled under comforters in the darkness at night.

So for the purposes of getting an idea of the system you might need, let’s start with the assumption that you’ll need something along the lines of 48-60 amp hours of battery capacity for each day. If you love reading by candlelight at night, you can lower your estimate. Have a borderline problem with with needing 3 margaritas whipped up in your VitaBlend Magnum 1800 XL? Maybe bump it a bit, but I will mention that our trailer has about a 0.7 amp “phantom draw.” That number accounts for the fridge DC power and the propane detector and means that even without turning on a single light, we use almost 17 amp hours each day so make sure that’s factored into your estimate.


Newer Airstreams (I’ll be mostly referencing Airstreams in this post since it’s all I know, but most of the info should be applicable to any other brand) come with two “Group 24” deep cycle sealed lead-acid batteries. These batteries are “maintenance-free” meaning that you don’t need to check the acid level periodically. The group number refers to the size and capacity of the battery.

As a starting point, consider that these batteries contain about 85 amp hours each for a total of 170ah, but it’s generally felt that running them below 50% of their capacity will dramatically reduce their lifespan. That means, Airstreams come from the factory with about 85 amp hours of usable battery capacity. Using our estimates of usage above, that means you’d be lucky to get through two days without a charge.

A few people have lately discovered that you can fit a Group 27 battery in the factory battery compartment located on the tongue of an Airstream with a little trimming of the lip that runs around the rim. (Note that for now, while I’m only addressing options for mounting batteries on the tongue, it is possible to add multiple batteries like these into externally vented compartments elsewhere or to look into AGM batteries discussed below.) These batteries have around 95 amp hours of capacity for a total of 190. Again following the 50% rule, you could count on 95 usable amp hours for a cost of about $120 each ($1.26/amp hour). That might get you through two full days at one of the lower usage estimates, but you’d need a charge as soon as possible.

Many, ourselves included, have opted to install dual 6V batteries, wired in series (I hope I have that right) to behave as if they are 12V. While installing these batteries does require a modification to your battery box, any welder or (in our case) someone with a chop saw and cut off blade and the McMaster-Carr app can do it for you for maybe $50. Trojan T-105’s are popular with a 225 amp hour capacity, although adding the second one doesn’t double your capacity, just gets you up to 12V and leaves you with about 112 ah usable. They’ll run you about $180 each ($1.60/amp hour). We opted for the Trojan T-145’s with 260 amp hours for more like $200 each ($1.53/amp hour). They’re a little taller, but it doesn’t really make a difference if you’re already modifying your battery box. Another interesting note, there’s some opinion out there that 6V batteries will hold up better under deeper discharge, meaning that two 6V’s at 260ah could be considered to have more like 150 or 175 ah of usable capacity without damaging them. For now, let’s assume 150 ah and mark them down as being able to last about 3 days at lower usage. And just so we’re clear, at this point, we’re assuming zero recharging from solar.

Now, if these numbers are not doing it for you, or you are daunted by the prospect of modifying your battery box, or you just have money burning a hole in your pocket, you can also go with AGM or Absorbent Glass Mat. These batteries have the advantage of not requiring venting, so they can be installed under a lounge or bed. The real disadvantage is price. You can get two Lifeline GPL-31X AGM 12V batteries with a total of 250ah (125 usable) for $400 each ($3.20/amp hour!) that will fit in your battery compartment or grab a single big Lifeline GPL-8DA for just a little less to put under the lounge. If you really need the big capacity, put four 6V Trojan AGM’s under your lounge for $1200 and you’ll have a total of 400 ah (200+ usable)but be careful about how much weight you’re adding to your rig.

The question really comes down to, how much is each additional day of non-reliance on external power (solar/hookups/generator) worth to you? From the factory (essentially “free”), you get about a day and a half. For $240, you can have two days. For $400 plus $50 to modify your battery box, you can have three days. For $1200, you can have four. As I mentioned above, we felt three days was a good place to start.


Now that we’ve covered usage of the storage capacity in your batteries, we can move on to recharging them. Recharging can be achieved through hookups at an RV park, running a generator or solar panels. Note, from what I’ve read, your trailer also receives about 8 amps through your umbilical from your tow vehicle while driving.

First a comment about charging at an RV park or from a generator. Your Airstream likely came from the factory with the standard two stage charger which unfortunately, tend to fry batteries by charging them for too long in “bulk” mode before falling into a maintenance “float” mode. Upgrading to a three stage charger adds an “absorption” stage which tapers voltage as the battery approaches capacity before transitioning to “float.” Some chargers even have a fourth stage of “equalization” that refers to a maintenance process that can refresh the batteries periodically by purposely overcharging them to knock deposits off the plates. Unless you plan to be solar *only* (in which case the solar charge controller is the only one you need to worry about), I would seriously consider upgrading to a 3 stage charge controller. Doing so will not only protect any investment you make in new batteries, but will result in much quicker charge times from a hookup or a generator.

Now, when your batteries are low, you can always pay for a night at an RV park. For $35 or so, you’ll wake up to topped off batteries and empty tanks. If this sounds like a good deal every two to three days (depending on the storage capacity you decided on above), read no further and enjoy your travels. But if you’d like the ability to stay out at that perfect boondocking site or even in the National or State Park site you snagged for the holiday weekend, you’ll need to consider portable power.

For about $700, you can pick up 1000W Honda or Yamaha generator that will be relatively quiet and completely reliable. One gallon of gas will last you something like 8 hours of run time, and assuming you’ve upgraded your charge controller, you won’t need more than two hours a day to top off your batteries. Camping in hot climates? Two Honda eu2000i generators ($1000 each) can be linked together to provide enough power to run your air-conditioner while charging your batteries at the same time. Sure, they’re a bit bulky, and you’ve gotta deal with getting gas for them now and again, but if this doesn’t sound too bad to you, you too can log off and head out on the road.

But you came here to read about solar, which means you’re interested in a way to recharge your batteries either more quietly, more ecologically or just more conveniently. And let’s go through what you’d need to do that, but just keep in mind, that solar does not have to be an all or nothing proposition and may just end up being a part of your long term solution in support of a generator or occasional RV park.

Solar panels come with wattages which, when divided by their voltages less some expected loss, give you peak amps they can collect. A 100W panel, for example, can collect up to around 6 amps. A 135W panel might give you closer to 8 amps. Now remember all those numbers for our amp usage? Here’s where we try to put those amps back into our batteries. A panel collecting 8 amps each hour for four hours might return 36 amp hours into our battery banks. If we could count on that everyday, for every two days of sun, we’d gain more than a day and a half worth of battery capacity. Day 1: 48 ah used + 32 ah collected = 16 ah down. Day 2: 16 ah down – 48 ah used + 32 ah collected = 32 ah down. Day 3: 32 ah down – 48 ah used + 32 collected = 48 ah down. Day 4: 48 ah down – 48 ah used + 32 collected = 64 ah down. Day 5: 64 ah down – 48 ah used + 32 collected = 80 ah down.At low usage estimates, we’ve just made our factory 85 ah battery capacity last five full days instead of just under two.

Unfortunately, conditions are not always perfect. Sun angle can be low during seasons other than summer, and your trailer will not always be perfectly aligned with the sun’s path. Plus, it’s nice to have a little shade in your campsite, but each hour of shade means one less hour of collection. A portable panel usually consists of two sides that fold like a briefcase and can be positioned away from the trailer and more effectively directed at the sun. In practice, we’ve found that a smaller portable panel can often out-collect our larger roof top panels simply by being in the right place at the right times.

The question then for sizing your own solar panel system is, how long would you like to extend your stay? Would another day get you through the long weekends? Are you planning for four day music festivals? Or would you like the ability for your solar system to enable you to stay indefinitely in a single location? If the latter, you’ll need enough wattage to collect enough amps over a reasonable-to-expect period of sun in a given day. For the purposes of estimation, I’m using a very reasonable year-round number of 4 hours of peak sun.

A few examples:

Usage Watts/Amps Hours Collected
48ah 200W/12A 4 48ah

This would get you there at a low usage level and assuming good strong sun, not always possible.

Usage Watts/Amps Hours Collected
60ah 300W/18A 4 72ah

Better, and would account for higher usage with a little room for some clouds.

Usage Watts/Amps Hours Collected
60ah 405W/24A 4 96ah

Ah, that feels better to me and SURPRISE SURPRISE, this is the set up we ended up going with. This also means that should we encounter those dreaded three days of rain/eclipse and find ourselves down 180 ah, with each day of sun, we’d have a net gain of 36 amp hours (96 ah in, 60 ah out) and could be back to 100% in 5 days, less with longer periods of sun.

Okay, I’ll admit we actually have a little more cushion in the form of our 120W portable panel which brings our total collection ability up to about 525W/30 amps but I’ll talk a little more about that under my final recommendations.


I’ve already made a few above, and if you’re happy with the idea of plugging in or firing up a generator, save your money ‘cause even in the long term, solar’s probably not the cheap route. But it is convenient. I love not having to worry about when and if to fire up a generator. I love not having to coil and uncoil power cables at a park if I can avoid it. I’m not exactly sure where we’d put a generator in our SUV if we had one (but we’ll figure it out if we decide we need one at some point). I love knowing that we’re collecting 20+ amps just cruising down the highway on a sunny day and can likely expect to arrive at the next awesome camp fully charged. So with that said, I thought I’d leave you with a few ideas on how you might get started on the solar path.

Based on our experience and what we end up finding effective, the first thing I’d recommend is getting yourself a portable panel, 120W or more if you have a place you can store it in your tow vehicle. (We built a small riser, think of a wide flat plywood garage, in the back of our SUV that the panel slips under in its case with storage bins piled on top.) We lasted for a couple years of weekends and some longer trips in our 19’ trailer with the stock Group 24 batteries with just our 120W panel (see a scenario like the first example above in which two days of capacity became five). And despite the $500 or so price tag, the reason I say to get the portable first is a) you might just find you don’t need anything else and 2) even if you mount panels on your roof, you’ll still want the portable for shady campsites and optimal positioning. 

The best next step at this point would actually be to spend about $200 on a battery monitor. We have the IPN Pro Remote which came with our solar kit, but there’s no reason you can’t buy it on its own right now to get a better idea of how much power you’re using and how much solar you’re collecting. Its readout shows you your amp deficit or surplus at any given time and will keep a running tally of battery capacity, amp hours down and voltage. Whether or not you end up with rooftop panels, you’re going to need this and having it might just convince you you don’t need them at all!

Third, I would suggest upgrading your batteries, particularly if your stock batteries are more than a few years old and getting a little “tired.” The Group 27’s are some improvement for a reasonable cost, but personally, I would suggest biting the bullet and going for something like the T-145’s. Three days of overcast weather doesn’t really seem all that unlikely to me, and I’d like the ability to stick it out for that long if need be. If the idea of cranking up stabilizers and hitching up to find hookups somewhere after a couple days of rain doesn’t sound too bad, don’t spend the money or hassle with modifying the battery box. If three days isn’t enough, either fork over the cash for more batteries or buy a generator. But on the contrary, if three days seems reasonable, with this setup and your 120W portable panel collecting 7 amps for 4 hours on some sunny days, you could extend your stay significantly. I’ll spare you the math, but it would likely outlast your gray and black tanks.

After these three steps, if you find that you need the additional collection capacity to recover from those overcast days or short winter days, call the guys at AM Solar and order yourself one of their “system cores” which include a charge controller, combiner box, cable, fuses and shut off switch. Without the remote battery monitor (which you already have), they should be in the $500 range. Call them to talk over the options. They are very knowledgeable and helpful.

Now decide on what you need on the roof. Remember the example I was so happy with above? That was 405W total. Since you already own a 120W portable panel, you might go with three 100W panels or even two 135’s or 160’s. The best part about this approach is that if you find you want more collection capacity in the future, it’ll be very easy (assuming a little pre-planning with regard to panel mounting locations) to add additional panels and wire them into the four port combiner box. With 300, 270 or 320W total on the roof, on the average day of good sun, you’ll be collecting in the ballpark of 18 amps an hour and be topped off in under 4 hours while never having to pull out your portable panel. But when conditions are less than ideal or that perfect spot is in the shade, your portable, positioned perfectly for 6 hours, will pull in 42 amp hours, almost enough to do the job on its own. With everything together and well positioned in the sun, you’d be full in a little over 2 hours.


We’re actually very happy with the system we have - 260 ah of battery capacity (150+ usable), 405W panels on the roof and a 120W portable, but I actually think we have more than we need on the roof. Sure, I’d love a little more battery capacity to weather a few more cloudy days, but I’m not sure it would be worth the cost, weight or reconfiguration that would be needed in our case. Honestly, if we find that we come up against that three day barrier frequently and don’t want to be forced to move camps, our next step will be to invest in a propane converted 1000W generator. Heading to the Pacific Northwest over the next couple early Spring months is likely to answer that question for us pretty quickly, and I’ll be updating here as we go!

Hopefully some of the above has been useful to you in your decision making process. Feel free to get in touch if you find any errors or have any additional questions I might be able to help with or direct you to someone who can!