# Help design my solar system for living fulltime in a travel trailer

Registered Users Posts: 40 ✭✭
Hello everyone. I'm new to this forum, and RE in general. I am custom building a 20' travel trailer for my girlfriend & I to live in fulltime. We plan to spend a lot of time in the south west United States & would like to take advantage of boondocking as much as possible, hence the need/want for solar battery charging.
So.. I think the best place to start is at the battery bank, considering space & weight capacity. I would like to use four 6v T-105 batteries at 225ah each. Connected in series/parallel to achieve 12v @ 450ah. This should give plenty of storage for our needs ( roughly 100ah a day )
Am I right in figuring I need about 850w of panel array? Or can I get by with less, maybe 400-600w?
«13

Good Morning Kevin,

Some basic math as a start... Recommend 5% to 13% rate of charge for solar power--5% is "OK" if you are doing weekend/summer seasonal boondocking... 10%+ if you are full time off grid. So:
• 450 AH * 14.5 volts charging * 1/0.77 panel+controller derating * 0.05 rate of charge = 424 Watt array minimum
• 450 AH * 14.5 volts charging * 1/0.77 panel+controller derating * 0.10 rate of charge = 847 Watt array nominal
• 450 AH * 14.5 volts charging * 1/0.77 panel+controller derating * 0.13 rate of charge = 1,102 Watt array "cost effective maximum
Next, based on amount of sun and power you use... 100 AH per day:
• 100 AH * 12 volts = 1,200 WH per day
Then based on amount of sun and seasons... Note that most RV panels are mounted flat to the roof... You can do better if you tilt the panels. You can compare output vs tilt on the following link (summer in US Southwest does not matter as much--Winter can be a different story--Near the coast, clouds and marine layer vs high desert--affects your harvest too):

### Los AngelesAverage Solar Insolation figures

Measured in kWh/m2/day onto a horizontal surface:  Jan Feb Mar Apr May Jun 2.93 3.62 5.12 6.60 7.49 7.83 Jul Aug Sep Oct Nov Dec 7.54 6.87 5.70 4.45 3.34 2.73
Toss bottom three months (generator backup for bad weather, etc.):
• 100 AH * 14.5 volts charging * 1/0.77 panel+controller derating * 1/3.62 hours of sun = 520 Watt array "break even February"
Still have lots of choices to make--Charge controller, wiring, breakers, AC inverter, etc...

-Bill
Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
• Registered Users Posts: 40 ✭✭
Awesome. Thank you Bill. That is exactly what I was hoping to get, some formulas & a little guidance. So I think I'm on the right track. When I get some time later I will plug away at some numbers.
I plan to use an mppt charge controller but do I base the amps from say 10% of my battery bank (45A) or do I take my panel wattage(855w) divided by charging voltage(14.8), giving about 72amps which includes 25% safety margin? I know to not exceed the input voltage of the controller. The amperage has me confused. By the way Trojan specs 14.8v for absorption charge. Should that be the number I use instead of 14.5v?
Just rough numbers. 14.5 vs 14.8 volts is not going to change much (about 2% difference). Some of the time you will be charging below 14.8 volts (battery less than ~80% full, other times, you will be charging closer to 14.8 volts (battery ~80-100% full).

Trying to give you reasonable numbers. Solar power, if your various calculations are within ~10%, then there is pretty much no difference.

For an RV with wide temprature variations--Getting an MPPT controller that also has remote temperature sensor I would highly recommend. Also, if you plan on charging >13% rate of charge, a RBTS can help reduce the chances of battery thermal run away (battery gets hot, charging voltage falls, charger outputs more current, battery gets hotter, etc.).

-Bill
Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
• Solar Expert Posts: 6,006 ✭✭✭✭✭

You haven't discussed loads? the Southwest is mighty hot, do you plan on using an AC? Campers and AC and solar don't play well together.

Home system 4000 watt (Evergreen) array standing, with 2 Midnite Classic Lites,  Midnite E-panel, Magnum MS4024, Prosine 1800(now backup) and Exeltech 1100(former backup...lol), 660 ah 24v Forklift battery(now 10 years old). Off grid for 20 years (if I include 8 months on a bicycle).
- Assorted other systems, pieces and to many panels in the closet to not do more projects.
• Registered Users Posts: 40 ✭✭
Bill. I will definitely make sure i have the ability to tilt the panels. Also will install remote temp sensor. I just read somewhere else about "battery theremal runaway". Thanks for the tips
Do you have any info to sellect the proper mppt controller for my situation? How do I determin the amps needed? 45a -60a -80a etc...
Photowhit. I appreciate you pointing that out. I do not plan to run air conditioning from the batteries. I will probably chase the seasons to try and stay in "tolerable" weather. A generator is also on my list of must haves although I would like to limit its use as much as possible. Then the last ditch option for me will be to pack it up & head for a plugin campsite:)
Thanks guys, please keep the insight coming. There is still so much I don't know, trying to take it one piece at a time. I'm still a long way before I even think about buying anything.
• Registered Users Posts: 40 ✭✭
Does anyone have a good book reference that is fairly thorough & not just a beginner's crash course?
• Registered Users Posts: 40 ✭✭
Btw. Sorry if this has all been discussed before. It gets tiring sifting through all these threads on a smartphone. Please feel free to point me to another thread if necessary. Thanks again
edited April 2016 #9
For smart phones--In the US with standard data plans, (in theory, by law) you can setup your phone as a WIFI station (no extra charge)--And use a tablet or computer. Makes reading long threads a lot easier.

Reading material--You can take a look at this post:

http://forum.solar-electric.com/discussion/comment/99323/#Comment_99323

In general, the quickest way to understanding designing your solar system--Just keep asking questions in your thread (a couple at a time--Do not ask 20 questions at once, it gets confusing really quickly--And a wall of questions is hard to even reply to).

Don't worry about asking too many questions--Everybody here started at ground zero and folks here will help when/where they can.

My suggested way to work through the solar install--Follow the leader... For example, I posted about your (guessing) location, and made some guesses about system size/usage. You want to tilt panels and may be using the system for full time off grid? Verify the numbers (do you have room for xyz watts of solar panels, will your battery bank be large enough for your needs, can you fit your battery bank, can you afford/justify thegenric costs so far, etc.).

Once you have the paper design basics... Then you can start selecting hardware. Solar panels and batteries are pretty generic in pricing. solar Charge controllers have a wide base of pricing/function/costs. I could suggest a \$600 full function MPPT charge controller, and you are really looking at a \$100 PWM charge controller--Give us some guidance what you are looking for (lots of computer integration, data logging--Or want to try an inexpensive Chinese \$100-\$200 MPPT controller and see how it works).

Also, there are choices to be made... For example, larger solar panels (say >200 Watts) are 60 or 72 cell panels which cost ~\$1 per watt (cheap) and need (expensive) MPPT charge controllers to match Vmp-array to Vbatt voltage.

Or, you can use ~140 Watt panels at ~\$2 per Watt (not cheap) with a cheap PWM charge controller. For RV's, because of their small size (not much roof space), the smaller panels may fit better and a cheaper PWM controller is fine (small system). You usually do both paper designs and figure out the total costs and functionality (of charge controller) and decide which is best for you.

So, before I "answer" your charge controller question--Can you tell me how many Watts of solar panels you will be using, what panels you are interested in using (smaller 140 Watt panels or larger 200+ watt panels) to fit your roof/pocket book?

-Bill
Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
• Solar Expert Posts: 175 ✭✭✭
Bill, BB, has given you good pointers on the math side. Here is a reasonable reference from an RV'er on the boondocking side.
http://www.jackdanmayer.com/rv_electrical_and_solar.htm#The%20Golden%20Rules . I prefer to use 12.4 volts rather then 12 and 14.8 rather then 14.5volts as Bill said doesn't make a big difference but I feel it will get you closer in approximate wattage you will need. This will let you get a better idea of physical panel size and the amount of roof space it takes up. If you are going to be mostly in the southwest without much shading on the panels, series maybe better but will require mppt for the controller; if on the other hand you will be in a lot of shade then consider parallel panel wiring with a portable placement method. We went with two roof mounted, tilt able and two portable. We also have half the battery bank you are planning so that lets us get away with 360 watts of panels and a PWM controller. We are also very conservative in our power usage. No T.V. or high wattage items like microwave or drip coffee maker.
Remember that 14.5 volts in the equation is an estimate of average charging voltage over time. Used to estimate solar harvest numbers only.

Not a charging set point.

-Bill
Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
• Registered Users Posts: 40 ✭✭
Thanks Everyone for the input. Much appreciated. I have been working long hours lately & haven't had time to do anything else. I'm going to try to figure some of this out tonight if I can stay focused. Hopefully I can give more details soon
• Registered Users Posts: 40 ✭✭
For what it's worth. I am building my trailer from scratch. The trailer I bought was severely water damaged so I tore it down to just the steel frame. So far I've only got the floor done, no walls or anything else yet. There will be plenty of room for panels on the roof. Just two vent fans & fridge vent will be up there, probably some sort of antenna too but no a/c on top. I know, I'm a long ways away but just trying to get a grasp on the solar aspect now so when the time comes I will be ready. Also we plan to visit many places not just the southwest but I do see us spending some time out there especially in the winter.
• Registered Users Posts: 40 ✭✭
Hello, everyone.

I'm back with more questions for anyone willing to give some advice.

I'm thinking about using three Suniva optimus 285w mono panels (855w total). I'm guessing since they have 60 cells & the voltage, this means 24v panels. Am I right?

Open circuit voltage (Voc) 38.8v
Max power point voltage (Vmpp) 31.8v
Short circuit current (Isc) 9.48a
Max power point current (Impp) 8.98a
Series fuse rating 15a
No PTC - What does this mean??

Anyone here have any positive or negative experience with this brand of panels?
Still looking at a 450 AH @ 12 volt lead acid battery bank?

Vmp=32 volts is not a Panel that can be directly connected to a 24 volt battery bank.

You really need 35-40 volt Vmp panels to do that.

At this point, you want the panels in series (Vmp-array=32 volt) and one or two mppt charge controllers.

That large of wattage array on a 12 volt battery bagel may need two charge controllers. Using one Midnite Classic charge controller may not be enough (running a mppt controller at 100% rated output in hot weather/room is not a first choice).

-Bill
Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
• Solar Expert Posts: 3,741 ✭✭✭✭
kevin85 said:
I'm thinking about using three Suniva optimus 285w mono panels (855w total). I'm guessing since they have 60 cells & the voltage, this means 24v panels. Am I right?
With 855 watts of panels you are really getting into the range of a 24 volt battery (and MPPT controller).

As Bill mentioned, that panel voltage is not adequate to charge a 24 volt battery.  Two of them in series (with an MPPT controller) would work very well with a 24 volt battery, but then you need either two or four of them.  Putting three panels in series (with a 24 volt battery) is workable, but not ideal.

If your battery is 12 volts, the three panels must be in parallel and you must use a large MPPT controller.

--vtMaps
4 X 235watt Samsung, Midnite ePanel, Outback VFX3524 FM60 & mate, 4 Interstate L16, trimetric, Honda eu2000i
• Registered Users Posts: 40 ✭✭
Yes, I still want to use a 450ah @ 12v battery bank. I was thinking that if those are 24v panels then an mppt controller would step this down to 12v quite efficiently. That's what I have concluded from other readings.
• Registered Users Posts: 40 ✭✭
I agree, it seems like I'm going to need a very large controller. I was hoping something like a morning star 60a mppt would work but now I'm having doubts.
Maximum "cost effective" array on an 855 Watt array (that I would suggest) for a 12 volt battery bank:
• 855 Watt array * 0.77 panel+controller derating * 1/14.5 volts nominal charging = 45 Amps
So a 45 Amp is "close", but a 60-80+ Amp MPPT controller should be fine.

Somewhere in my thinking (and not writing it out)--I thought you were doing a 2x larger battery bank instead of the 450 AH @ 12 volt battery bank....

-Bill
Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
• Solar Expert Posts: 3,741 ✭✭✭✭
kevin85 said:
I agree, it seems like I'm going to need a very large controller. I was hoping something like a morning star 60a mppt would work but now I'm having doubts.
As Bill pointed out, under typical conditions your 855 watt array is putting out 658 watts (77% derate) and a 60 amp controller would be adequate.   Of course, in very cold climates you could put out a full 855 watts and if your battery were only at 12 volts (early bulk charge), you could put 71 amps into the battery... higher than the rating of a 60 amp controller.

Fortunately, the morningstar controller will not be harmed by occasional "overpowering"... it will clip the excess power.  Since you are in a trailer, I think it would be worthwhile to avoid the higher capacity controllers that have noisy fans.

You can see the issues with trying to handle 855 watts with a 12 volt system... that morningstar would not even notice 855 watts on a 24 volt system.

--vtMaps
4 X 235watt Samsung, Midnite ePanel, Outback VFX3524 FM60 & mate, 4 Interstate L16, trimetric, Honda eu2000i
• Registered Users Posts: 40 ✭✭
Thanks for the quick response BB & vtMaps.

Vtmaps nailed what I was concerned about. The cold temps pushing voltage to its peak ~70+ amps.

True, I was trying to avoid the larger controllers with fans. If you guys think I would be ok with the 60a morning star then that's the ticket. Would I be loosing potential power by going this rout? Would I be pushing the controller to hard?
The Morningstar controllers have been (on average) pretty reliable products. You could always try one 60 amp MPPT controller and monitor its output. If it is clipping "too much for you" in sub freezing weather--Then you could always split the array and get a second 60 amp (or possibly 45 amp for a single panel) controller and wire them both to the same battery bank.

In general, solar panels heat up in full sun--So even on days "cool days", the panels will not be outputting that much higher power. It takes really cold weather and/or edge of cloud events to get the array output that high. And, in many cases, those events are not that long (seconds to minutes).

Any chances you would add a fourth (or more) panels?

-Bill
Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
• Registered Users Posts: 40 ✭✭
On another note. Would it be advantageous to wire the same four 6v batteries in series @ 24v and use a 24v to 12v converter insted of 12v battery bank? Please keep in mind cost effectiveness. I'm on a tight budget but firmly believe in doing it once the right way, even if it costs a little more upfront you save money in the long run.
edited April 2016 #24
A 24 to 12 VDC converter is not that much different than a 120 VAC current (close to the same losses).

If you go to 24 VDC + Inverter, you can now run 120 VAC appliances (or 120/240 VAC).

The DC to DC converter, if your loads are small (few amps)--Then a small DC converter may be the cheaper way to go. Really depends on your power needs at 12/24VDC and 120/240 VAC.

Note that large AC inverters do suck up extra power--And adding a 4th panel may be nice to do eventually (by next fall/winter especially?).

-Bill

PS: Thank you vtMaps--Fixed a few above to VAC (not VDC). -Bill
Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
• Registered Users Posts: 40 ✭✭
BB, I am willing to add one more panal for a total of four (possibly portable) but would prefer not to unless necessary. Nothing, as of now, is set in stone. I can choose different panels, controller, etc. I'm just trying to figure out the best system using 4 - 6v 225ah (all I can fit) batteries pulling roughly 100ah load each day
• Registered Users Posts: 40 ✭✭
By the way I do have 120V ac loads & was looking at the xantrex prowatt sw2000w true sine wave inverter
• Registered Users Posts: 40 ✭✭
2000w only for microwave & hairdryer spikes
OK, thank you guys... Fixed the VAC typos.

With Microwave and Hair dryer... That is getting pretty up there. With good batteries and short/heavy wiring, you can run ~2,000 Watts with a good inverter (the Xantrex 2000 Watt TSW Pro Watt has been a good product, from reports here).

HOWEVER--For folks that live in cold regions, the 15 Volt High Voltage Shutdown could be a problem--You may see 16.5 volts when charging/equalizing a cold battery bank.

And just to give you an idea... The 2000 ProWatt runs about 0.8 amps (just turned on). Or:
• 13.0 volts * 0.8 amps * 24 hours per day = 250 WH per day
• 0.8 amps * 24 hours = 19.2 AH per day
For your system--That was around 20% to 25% of your planned loads... Not a killer, but adding an extra panel for winter/if you plan on running the inverter 24x7 can be nice. (although, most of the year, your 855 Watt array is going to do pretty well even with an inverter running 24x7).

And, if you run the inverter 24x7--That means that many of the "DC loads" (LED lighting, radio, computer, etc.) could be pretty happy at 120 VAC even with the "extra tare and operational losses".

-Bill
Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
• Registered Users Posts: 40 ✭✭
Thanks again for the input BB. I'm always happy to read your responses. My ac loads are all pretty much optional and for short periods. The inverter would only be turned on when needed. Do you think that the xantrex would still be a good choice? I can give rough estimates of my ac & dc loads if that would help. Again I don't live in it yet so these loads are an estimate
• Registered Users Posts: 40 ✭✭
Also I do plan to shy away from subfreezing temps. Although there are some colder destinations in my future, I would probably visit them in the warmer months
The loads are "what matter"... If the system does not cost effectively power those loads, then it would not meet your needs.

I do not run off grid, and I am not in the solar business--I will leave it to others to comment on good inverter/hardware options. But, from what I have read before on the forum, the Xantrex Prowatt was well liked.

The choice between 12 volt and 24 volt battery banks--Many people here have started at the lower voltage, and then wished they had gone at least one step up--As their loads (and system) kept growing.

Power usage is a highly personal set of choices... I cannot substitute my "beliefs" and choices for your yours. You are the only one, in the end, that can make these decisions.

If you are using a 450 AH @ 12 volt set of flooded cell lead acid batteries--More or less, ~1,000 Watts is about the maximum continuous AC Watts that you can draw from that size bank (minutes to an hour or so--C/5 discharge rate).
• 450 AH * 12 volts * 0.85 AC inverter eff * 1/5 discharge rate = 918 Watts @ 120 VAC (short term max AC load)
• 450 AH * 12 volts * 0.85 AC inverter eff * 1/8 discharge rate = 574 Watts @ 120 VAC (suggested multi-hour AC load)
If this is your battery bank's maximum planned size (and you don't go to AGM/Li Ion, etc.)--You really only need about a 1,000 Watt maximum inverter. (and not much more than ~1,110 Watts of solar panels either--To much charging current can be a problem for lead acid batteries too, as well as not really cost effective unless you have a lot of day time loads).

-Bill

Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset