Seeking advice for solar and battery -- fixed-site off-grid Airstream trailer

paulbmccarthypaulbmccarthy Registered Users Posts: 3


My wife and I recently bought 3 acres of agricultural-zoned land in a beautiful area of coastal California less than a mile from the beach. Part of the reason it is still such a pristine and beautiful area is because of the VERY intense regulation of building in the area. One day we may build a house there (it is legally allowed), but we don't have the money for that now --and the red tape involved in that process will be insane and very very slow (coastal commission of CA is notorious).

But of course, we can put a mobile home there, legally, "in storage". My wife and I recently bought a 28' 2005 Airstream Int’l CCD, and we are planning on parking it full time on the land as our weekend getaway spot. It’s less than an hour’s drive from our house so we will be going there almost every week. We really don't plan to travel with the airstream, at all. We don’t even have a tow vehicle (we’re hiring someone to deliver it). I know that will shock some of you. We can do composting toilet or some other creative ways to deal with blackwater (don’t ask). We have a water well on-site, but no power. It will take a LOT of money to hook up power to the grid for the site, so we are hoping to design a solar battery system for the airstream instead. That’s where we would love your help.

I’ve been reading and researching for a long time on this site, and on other solar off-grid sites (and on some sailing sites, where they also deal with need for off-grid power) but I wanted to see what y’all think we should do, as far as a solar and battery system. The area is on a sunny hillside with a 13% grade facing east. From my research online it looks like the area gets 3.2 peak sun hours in December, and 7.7 peak sun hours in July. pvwatts puts it at about 3.4 kwh/m/day in December up to 6.75 in July. It’s a temperate area — average lows are around 42 in the winter and average highs are around 64 in the summer. So we will be running the furnace (runs on propane, but the blower uses electric), but then of course there is the water pump, the lights (we will replace with LED), the stereo system, fridge (runs on propane), propane detector, our laptops, fantastic & bathroom fans, phone chargers, and other stuff I’m probably not thinking of. Most people say this is about 40-60 Ah per day, it seems. I would like to design a system that can run without generator as much as possible, sized big enough so we don’t have to worry about occasionally running other random items (projector for movie night with friends! blender for smoothies the next morning!) — but we aren’t going to be running a hair dryer every day, or a microwave (I don’t think!). We aren’t likely to run the AC much, but the heat strip might be nice if we run out of propane — but since I know those items are extreme electric guzzlers, we were planning on having a generator as backup to help with that if necessary. But I really really hate the noise of the generator, even the nice newer ones. So I’d like to plan on no/minimal generator, if possible.


I’m already convinced of the need for a high quality inverter / charger, so I’ve already bought a Magnum MSH3012M as the heart of the system. I assume I should get the remote battery monitoring kit and remote control panel from Magnum to go with it. 


The problem with our system is that unlike most airstream users, we don’t plan on using a generator or hooking up to shore power, ever. Unless we really really need to fire up the generator every few weeks, just to get the batteries up to 100%, to keep them healthy. Is that necessary? In an ideal world, we would love to fire up the generator NEVER. After reading a lot about partial-state-of-charge issues with batteries (it kills batteries, except for Lithium), I’ve been leaning towards getting two Trojan IND9-6V batteries, which are flooded lead-acid batteries that are designed for serious solar use, and designed to withstand PSOC issues. That would give us 464Ah (but of course only about 250Ah usable, since we don’t want to go much below 50%). My impression is that these batteries would last 10-15 years or more — they are used in serious off-grid applications in Africa, etc.  But maybe that is serious overkill. I could do AGM but my impression is that even with Lifeline they won’t last nearly as long, and would about the same. Lifepo4 just seems to new to me to able to rely on them for 10-15 years. Normal wet cell batteries like T105s seem like they will be killed quickly by PSOC with our scenario (no shore power ever, rare to never generator usage). I know that with the wet cells there would be some maintenance involved, and they would need to be in a ventilated area — maybe even in a box built outside the trailer, right next to it.


So how many panels would I need to generate the needed amps to properly charge these batteries? I’m assuming I will get the 100w mono Renogy panels, since they seem best bang for buck, as far as a 12v system. Maybe I should be thinking about a smaller overall system, but with the goal of minimal generator usage, and also the goal of a lazy “set and forget” general approach (other than battery maintenance), oversizing things a little seems makes sense. We’ll mount our panels on the ground close to the airstream where they can get full sun and be tilted at an optimum fixed angle. I guess I will need some kind of grounding pole into the ground somewhere too.


I see there are some real pros doing this type of installation, but none that are super close to me. There are, however many marine electrical installers around the Bay Area here in northern CA. Should I be calling up some of those places to see about installation of this stuff? I’m handy, and pretty good at following directions, but I don’t want to screw up the wiring on this.

Any advice appreciated.


  • vtmapsvtmaps Solar Expert Posts: 3,738 ✭✭✭✭
    Welcome to the forum,

    Stop spending money!  I am afraid the Magnum is not well matched to your needs.  You haven't told us enough about your loads to know if a 12 volt system is appropriate.  You might need 4 of those batteries configured in a 24 volt system.

    Tell us more about the blower on the furnace and the electrical specs on the well pump. 

    4 X 235watt Samsung, Midnite ePanel, Outback VFX3524 FM60 & mate, 4 Interstate L16, trimetric, Honda eu2000i
  • wellbuiltwellbuilt Solar Expert Posts: 479 ✭✭✭
    I am in a 73 airstream my self , I just use 2 gc230 ah battery and 210 watts of solar with a 40amp charge controller .
    Every thing runs on 12 v .
     I wired my trailer fuse box to the solar controller out put so I can't run the batteries down to 0 power .
     I have a very small 300/600 morning star inverter that mostly runs a Samsung 32" tv @22 watts with a USB thumb drive full of movies .
     I sleep with a c- pack 6/7 hrs a night  2 led out side lights , radio cell phone chargers water transfer pump to pump water from my 275 g tank into my trailer tank.
     I have never run out of power , I run a small generater and a cheap 30 amp battery charger .
     I would forget the large inverter it burns extra power . 
     I would just use a generator to pump water into a tank , my trailer has a 30 g water tank and we fill it every 3/4 days .
     A small 1/2 hp pump fills the trailer from the tank. 
     I have a deep well with no pump we get water off the porch roof . 
     400 watts of solar would be better .

    Out back  flex power one  with out back 3648 inverter fm80 charge controler  flex net  mate 16 gc215 battery’s 4425 Watts solar .
  • mike95490mike95490 Solar Expert Posts: 8,868 ✭✭✭✭✭
    Do you plan to haul the trailer out every week or so, to dump the black water, or have a pumping service come out ?   That's going to be a issue.
     I'd suggest a ground mount array, maybe like a covered walkway or pergola.  put the batteries in a wooden box at the base....

    But you do need to include plans for a generator, and battery charger, for when the sun don't shine. And coastal marine layer may take a lot of those sun hours away..

    So for a wild guess, 12v furnace fan, internal 12V water pump, and prewired 12V lighting. you need a 12V system.
    OR if you have 120V wiring, you could use a 24 or 48v setup and run the trailer off an inverter (but be aware you may have to disconnect the house battery charger or it will both overcharge the house battery, and it's constant drain will flatten the outboard battery bank)

    But the load data for your fan, pump, lighting, entertainment system and microwave all need consideration before you buy anything.
    Powerfab top of pole PV mount | Listeroid 6/1 w/st5 gen head | XW6048 inverter/chgr | Iota 48V/15A charger | Morningstar 60A MPPT | 48V, 800A NiFe Battery (in series)| 15, Evergreen 205w "12V" PV array on pole | Midnight ePanel | Grundfos 10 SO5-9 with 3 wire Franklin Electric motor (1/2hp 240V 1ph ) on a timer for 3 hr noontime run - Runs off PV ||
    || Midnight Classic 200 | 10, Evergreen 200w in a 160VOC array ||
    || VEC1093 12V Charger | Maha C401 aa/aaa Charger | SureSine | Sunsaver MPPT 15A

    gen: ,

  • BB.BB. Super Moderators, Administrators Posts: 30,721 admin
    edited April 2016 #5
    Unless you are planning on 2 axis tracker and are in a sheltered in-land valley--The coastal marine layer can be a real killer (I grew up just south of San Francisco on the coast). SFO has an average sun of:

    Measured in kWh/m2/day onto a solar panel set at a 52° angle from vertical:
    (For best year-round performance)
    Jan Feb Mar Apr May Jun
    Jul Aug Sep Oct Nov Dec
    I would suggest a assuming that you will use the genset sometimes during the three months of winter--A minimum solar array assuming 4.26 hours of sun (February) would be a good starting point.

    If you do not run any large loads (refrigerator, A/C, electric heating, large well pump), a 1,000 WH per day (~83 AH @ 12 volts) is a pretty useful system (laptop, cell phone charging, LED lighting, some battery power tools). Get a 300 Watt TSW inverter from MorningStar.

    Normally, a nice size battery bank is 2 days of storage and 50% maximum discharge. That would work out to:
    • 1,000 WH per day * 1/12 volt battery * 1/0.85 inverter eff * 2 days storage * 1/0.50 max discharge = 392 AH @ 12 volt battery bank (4x 6 volt @ 200 AH golf cart batteries in series+parallel are nice).
    Next, charging current... Nominally 5% to 13% rate of charge is recommended. For a weekend place, you can get away with 5% rate of charge. 10% rate of charge (or more) is recommended if you are staying there full time (and better battery life):
    • 392 AH * 14.5 volts charging * 1/0.77 panel+controller derating * 0.05 rate of charge = 369 Watt array minimum (weekend)
    • 392 AH * 14.5 volts charging * 1/0.77 panel+controller derating * 0.10 rate of charge = 738 Watt array nominal (full time off grid)
    • 392 AH * 14.5 volts charging * 1/0.77 panel+controller derating * 0.13 rate of charge = 960 Watt array "cost effective" maximum
    And then there is array sizing based on energy usage... A suggested minimum array would be:
    • 1,000 WH per day * 1/0.52 off grid system eff * 1/4.26 Hours of Sun (Feb) = 451 Watt array for "break even" February
    Based on the above guesstimates, an array of 451 Watt minimum (weekend use) to 738-960 Watt (full time off grid).

    One thing to think about--Look at getting utility power. It may be difficult and expensive now, but in 5 years, it may even be more expensive and difficult. Solar power is not cheap--And if you plan on retiring there--At some point you may need more power/want less maintenance with solar--Or eventually need to sell the property--Having utility power may be helpful.

    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
  • paulbmccarthypaulbmccarthy Registered Users Posts: 3
    Thanks for feedback, all.

    It will definitely be a 12v system, since every single thing in the vehicle is 12v. I could skip the Magnum, but I want something that can be wired as an inverter to provide AC to all my outlets in the airstream, and I like that the Magnum hybrid can handle the genset at the same time as the battery, if I decide to run air conditioning. I know that the bigger inverter does burn power, but I have been told that I can't power all the outlets in the trailer if I use a smaller one. Hmmm... maybe not true?

    Sounds like 6-8 panels (100 watt 12v mono renogy, in parallel) would be a pretty good setup. And that would be enough to keep the Trojan IND9-6V batteries happy? Would I need to run the genset monthly just to make sure the batteries hit 100% every month?

    I'm waiting to hear back from PG&E for grid hookup estimates, but since the electric line runs will be at least 300 feet from the road, and since everything costs extra in this part of the world, I would expect installing grid service to cost at least $10-15k. And besides, it might not be possible to start til permits get approved, which could be 8 months from now, or longer. I don't want to wait a year to get camping.

    I have a plan for black water, don't worry. 

    Any other feedback or ideas please keep them coming. Thanks.
  • paulbmccarthypaulbmccarthy Registered Users Posts: 3
    Okay so now I'm thinking a tristar 60a MPPT controller with full size panels totaling 1kw. Charging a pair of Trojan IND9-6V batteries. I think that will do it nicely. I got PG&E estimates, and though they are rough estimates, they are indeed in the $10k-15k range. And I would have to wait at least a year to go through all the permitting. I'd rather go solar and do it now! Thanks for your help, all.
  • BB.BB. Super Moderators, Administrators Posts: 30,721 admin
    OK... As I understand, those are "industrial batteries" which probably should be charged at 10% rate of charge minimum.... Using their AH capacity:
    • 464 AH * 14.5 volts charging * 1/0.77 panel+controller derating * 0.10 rate of charge = 874 Watt Array minimum
    Also--For a larger solar array--Perhaps you want to look at the larger format "GT Panels" (Vmp~30 volt, ~200-300 Watts) + an MPPT type charge controller (MPPT controller because of the non-standard Vmp~18 volts of the panels you are looking at). A fewer large format panels are easier/cheaper to wire (you can installthe array further from your home/battery shed) and run at higher Vmp-array voltage.

    Regarding a how much power--From an electrical point of view, even a small AC inverter (300 Watt) can power all of your outlets.

    What matters is the size of the loads... If you are using a few LED lights (4-9 watts a bulb), cell phone charger (10 Watts), laptop computer (30-70 Watts), etc... You can see that all of them added up do not exceed 300 Watts.

    However, as soon as you use a large(r) AC load (120 VAC refrigerator, Air Conditioner, etc.)--You are now talking about some pretty large energy requirements. And a "smaller" 12 volt @ 400 AH battery bank may not cut it.

    As long as your average AC (and DC) power requirements are "small" and well known--This should be a very capable system that can keep you pretty happy--As long as you respect the maximum load capacity of the battery and AC inverter you choose.

    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
  • vtmapsvtmaps Solar Expert Posts: 3,738 ✭✭✭✭
    Charging a pair of Trojan IND9-6V batteries. I think that will do it nicely.
    BB. said:
    OK... As I understand, those are "industrial batteries" which probably should be charged at 10% rate of charge minimum.... Using their AH capacity:
    • 464 AH * 14.5 volts charging * 1/0.77 panel+controller derating * 0.10 rate of charge = 874 Watt Array minimum
    Those thick plate industrial batteries can take a long time to complete absorb... of course, you don't need to complete absorb every day, but each day that you don't complete absorb means that the next time you do complete absorb it will take even longer.

    4 X 235watt Samsung, Midnite ePanel, Outback VFX3524 FM60 & mate, 4 Interstate L16, trimetric, Honda eu2000i
  • sanitariusanitariu Solar Expert Posts: 33 ✭✭
    Just get lifepo4 and forget these formulas. Also if your panels are 60 cells you are not going to see benefit from MPPT. Do not go for lead-acid battery. It's dead technology and you are going to trow them in 4-5 years.
    4 X 240watt Luxor, Victron Energy Multiplus Compact 24 volt 1200VA, electrodacus solar controller, 8 x WB-LYP160AHA LiFeYPO4 3.2volt
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