New guy seeking help with system design for a bus

Hey everyone, new to the forum, and had a few questions. I'm currently in the process of converting a school bus to an RV to start traveling come spring. I wasn't quite ready to do the electrical system yet, but I came across some inexpensive panels and decided to just scoop them up and figure it out later. I was able to get ahold of 8x 245w panels for only $600, totalling 1960 watts. I don't think I need quite this much, but I grabbed 8 of them anyway. They are bosch na30119 panels, rated at 245w, 36.8 V, 8.25 Amps.

In my bus I won't be using a ton of stuff. Currently I plan on using my 10cu ft fridge, a 32-40" led tv, dvd player and maybe a N64. I'll have a few outlets for charging devices when needed, like phone or a laptop. I also might add a microwave down the line, but will be starting out with just my LP oven / range. I will also have a couple 12v items like led lights, water pump, and roof vents.

I'm trying to figure out the best way to wire up these panels, and what to do with my battery bank. I've been told I should run a 24volt battery bank with those panels, but not given much explanation as to why. I'm familar with 12volt systems from my days doing car stereos and alarms, but this stuff is a little different.

Thanks in advance for any help or recommendations!


  • mcgivormcgivor Posts: 2,205Solar Expert ✭✭✭✭✭
    When a refrigerator is involved the system moves Into a mid sized system, forget 12V nominal, move up to 24V, or better yet 48V, much easier to manage loads, speaking of which, this where the whole calculation should begin, loads, battery and the means to charge them, classical mistake buying panels before loads are established, but it is what it is, the learning process is about to begin, be patient and you'll learn a lot.
    1500W, 6× Schutten 250W Poly panels , Schneider MPPT 60 150 CC, Schneider SW 2524 inverter, 400Ah LFP 24V nominal battery bank 

  • WanderingGooseWanderingGoose Posts: 5Registered Users ✭✭
    edited September 14 #3
    The only reason I got the panels first were for the price. $75 for a 245w panel. Also, several other bus owners were picking up the same panels so I just went with it and grabbed some too. Not the ideal way to do it, but if I would have waited I would be spending more money most likely so I just went for it. Now comes all the learning and more money spending, lol.

    I will also have a minisplit AC in the bus, but for now my plan is to just run that on generator only as I've heard too much that AC on solar isn't very practical
  • mcgivormcgivor Posts: 2,205Solar Expert ✭✭✭✭✭
    So now you have panels, 245W would indicate 60 cell panels, witch would mean a MPPT controller is in order, choose a nominal voltage, above 12V, 24 or 48, hint higher is better. 
    1500W, 6× Schutten 250W Poly panels , Schneider MPPT 60 150 CC, Schneider SW 2524 inverter, 400Ah LFP 24V nominal battery bank 

  • BB.BB. Posts: 27,886Super Moderators, Administrators admin
    Higher voltage battery bank, less current in wiring... Say 1,500 Watt AC inverter (1.2 to 1.5 kW typical minimum for an AC energy star refrigerator):
    • 1,500 Watts * 1/0.85 AC inverter eff * 1/10.5 battery cutoff * 1.25 NEC wiring+breaker derating = 210 Amp DC branch circuit+breaker rating.
    At 24 volt bank, 1/2 the current (and much smaller wiring, better voltage drop tolerance). 12 volts can be done... But ~1,200 to 1,800 watts is about the max for a practical 12 volt system.

    Nice to get a good deal on the solar panels... Using 5-13% rate of charge for typical system, with 10%+ good for full time off grid system, the nominal battery bank AH rating would be:
    • 8*245 W panels (fit on bus?) * 0.77 panel+controller deratings * 0.10 rate of charge * 1/14.5 volts charging = 1,041 AH @ 12 volt battery bank nominal
    • 8*245 W panels (fit on bus?) * 0.77 panel+controller deratings * 0.10 rate of charge * 1/29.0 volts charging = 520 AH @ 24 volt battery bank nominal
    The above battery bank would be good for a ~1,300 to ~2,600 Watt AC inverter on a typical lead acid battery bank. Going larger AC inverter, your battery bank will not be very happy.

    Store the same amount of energy (power=voltage*current ... 2x voltage and 1/2 AH capacity = same power).

    10% rate of charge at 1,041 needs 2x solar charge controllers (~60-90 amps capacity per controller, depending on brand and model).

    And then there is where you will be camping, what season(s), and tilt or flat mount panels. The farther north, the more you need to tilt in winter to get good harvest:

    Greensboro North Carolina
    Average Solar Insolation figures

    Measured in kWh/m2/day onto a horizontal surface:


    Average Solar Insolation figures

    Measured in kWh/m2/day onto a solar panel set at a 54° angle from vertical:
    (For best year-round performance)

    Toss the bottom three winter months (need genset to help in bad weather), February would be the "break even" solar month, the average harvest would be:
    • 8*245 Watt panels * 0.52 end to end AC solar efficiency * 3.89 hours of sun (Feb) = 3,965 Watt*Hours per February day
    Note that for base line loads, count on using ~50% to 75% of daily harvest... 4kWH per day is enough to run a small/efficient home/cabin from solar with an efficient refrigerator, LED lights, water pump, laptop, LED TV...

    Energy usage is a highly personal set of choices... The Math above is a series of rules of thumbs/guesstimates/balanced design to estimate what a paper design could look like and how to do the math for your needs.

    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
  • littleharbor2littleharbor2 Posts: 1,027Solar Expert ✭✭✭✭
    One option I don't hear discussed much would be to run all the larger inverter driven loads on a 24 or 48 volt system. Any small 12 volt loads, ie; water pump, stereo and LED lighting can be ran on a separate 12 volt system. Granted it involves separate panel, controller and battery but you also have the redundancy of separate systems and no step down or battery tapping issues. I'm sure this approach isn't for everybody but it has served me well in Baja over the years.

    2.1 Kw Suntech 175 mono, Classic 200, Trace SW 4024 ( 15 years old  but brand new out of sealed factory box Jan. 2015), Bogart Tri-metric, 700 ah @24 volt AGM battery bank. Plenty of Baja Sea of Cortez sunshine.

  • EstragonEstragon Posts: 2,839Registered Users ✭✭✭✭✭
    I also do the two voltage thing. It works well for me, but does add cost & some complexity.
    Main daytime system ~4kw panels into 2xMNClassic150 370ah 48v bank 2xOutback 3548 inverter 120v + 240v autotransformer
    Night system ~1kw panels into 1xMNClassic150 700ah 12v bank morningstar 300w inverter
  • PhotowhitPhotowhit Posts: 4,703Solar Expert ✭✭✭✭
    Not sure about your intentions in terms of how mobile you will be? If you intend to be traveling often, or drive and boondock for weeks at a time. If you will be driving regularly, you could run a separate 12 volt battery off you motor for small loads. or just do a 24 - 12 volt DC - DC converter.

    Your panels should be discussed at the working voltage and amperage as is their wattage. So the VMP IMP numbers used. When looking up the correct numbers to use I discovered your panels are under recall and I guess will be replaced;

    With nearly 2000 watts running a mini split may well be reasonable, though you would likely want a pretty good sized battery bank, or expect early demise from bringing a battery bank down too much. Even with a small area, you should consider sectioning off a smaller sleeping area making it easier on the air conditioner. Particularly since it will need to be in the sun.

    Home system 4000 watt (Evergreen) array standing, with 2 Midnite Classic Lites,  Midnite E-panel, Prosine 1800 and Exeltech 1100, ForkLift battery. Off grid for @13 of last 14 years. 1000 watts being added to current CC, @2700 watts to be added with an additional CC.
  • WanderingGooseWanderingGoose Posts: 5Registered Users ✭✭
    edited September 14 #9
    Lots of great information, some that I need to research the technical terms a little bit more to fully understand, but overall very helpful so far.

    I will be full time boondocking, but don't plan on traveling constantly. Plans could change depending on where I am and what's going on, but I dont expect to driving more than once or twice a month, probably for no longer than a day or two at a time. Most of the time will be in new mexico or near, but I do plan on seeing every state at some point.

    So far it seems like the 24volt battery bank is looking like the best way to go, with around 2000w 24v inverter. I also had already read that the MPPT charge controllers were the better way to go so I planned on going that route as well. And yes, all 8 panels will fit on the bus. It's 38ft bumper to bumper, about 31-32ish feet of roof space.
    I was also thinking of going with a 24 - 12v converter to run the 12v loads if I go with a 24v bank. I didn't have any intentions of wiring it to the vehicles system to let the alternator help with recharge, I'll have a portable genny if needed. It may be a good idea to figure out a way to let the solar keep the bus batteries charged though

    I did see that the panels were recalled, which is why they were being sold cheap I believe. I did not find out about the recall until after the purchase because I didnt research them well enough. I do know that it was a voluntary recall due to a small number of panels displaying what appeared to be burn marks (might be using wrong term), leading them to believe their is potential that the panels could short / burn up, but no incidents were ever reported. I did inspect all these panels and found no issues anywhere on them. I don't know if bosch would swap out these panels under the recall since I bought them on craigslist.
  • BB.BB. Posts: 27,886Super Moderators, Administrators admin
    Your panel numbers do not quite "add up"... Vmp and Imp are what we need... You may have Voc and/or Isc mixed in there (mp=maximum power; oc=open circuit; sc=short circuit).

    One of the issues with purchasing hardware before you need it--Sometimes you end up with a mix of equipment that does not really play well together... Your panels will work fine if configured for 2 in series, then 4 strings in parallel for a 24 volt battery bank (Vmp-array~60-72 volts standard test conditions). However, if you were to go with a 48 volt battery bank, you would need 3 of your panels in series... 2 parallel strings is 6 panels total, 3 parallel strings is 9 panels total (and you have 8 panels...).

    Regarding these panels being recalled. I had some from another vendor that "went bad" (problems in the J-Box caused overheating after about 5 years of use, needed to replace the entire 20 panel array). The overheating/failing connections made a "light coffee colored stain/haze" under the top glass just over the j-box on the back of the panel. You had to look pretty closely through the glass over the J-Box to see the failure indications--But once you saw one failure point--The others were pretty easy to see.

    You can check your panels with a meter set to 10 amps full scale current (or use a DC Current Clamp DMM) and put the panels under full sun in middle of day... If the output is >50% to 100%+ of Imp--They are currently OK (full near noon sun, facing directly towards sun). Since you have 8 panels, check all of them at the same time (within minutes of each other). They will either have Isc all about the same, or you will see one or more panels with 1/2 the current flow (bad panel).

    Whether your panels will fail like mine did (their output current dropped), or if a panel catches fire (could happen--Not sure it is a likely event)--Eventually one or more of them will probably fail over time.

    See if they can be replaced under warranty... Some vendors would do that.

    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
  • mike95490mike95490 Posts: 7,902Solar Expert ✭✭✭✭
    In my bus I won't be using a ton of stuff. Currently I plan on using my 10cu ft fridge, a 32-40" led tv, dvd player and maybe a N64. I'll have a few outlets for charging devices when needed, like phone or a laptop. I also might add a microwave down the line, but will be starting out with just my LP oven / range. I will also have a couple 12v items like led lights, water pump, and roof vents. ......
    That's actually a ton of stuff , if not a tad more !   It's fine with 3,000 w of PV. but you are not quite there, and bus roofs are seldom at the correct angle for good harvest.
    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: ,

  • oil pan 4oil pan 4 Posts: 751Solar Expert ✭✭✭✭
    Does the original bus electrical system 12v or 24v ?

    Solar hybrid gasoline generator, 7kw gas, 180 watts of solar, Morningstar 15 amp MPPT, group 31 AGM, 900 watt kisae inverter.

    Solar roof top GMC suburban, a normal 3/4 ton suburban with 180 watts of panels on the roof and 10 amp genasun MPPT, 2000w samlex pure sine wave inverter, 12v gast and ARB air compressors.

  • WanderingGooseWanderingGoose Posts: 5Registered Users ✭✭
    Okay, so I've been looking into some more stuff, and have a little better understanding. I've been looking at solar charge controllers and it seems like my choices are either a real expensive 80amp or run two controllers due to having 1960w pv input. This is the setup I'm currently looking at, and am looking to purchase in the near future.

    Running my 8x panels, running pairs in series, then parallel, 4 panels going into each charge controller and paralleling the charge controllers to the batteries
    For the gear I was looking at sungoldpower 24v 3000w inverter/charger, and two 40 amp epever mppt charge controllers.

    Do yall see any problems with this setup? Or reccomend changing anything? Made those gear selections because I'm on a budget and still need to buy the batteries too
  • petertearaipetertearai Posts: 348Solar Expert ✭✭✭
    Consider going to 48 volts .
    2225 wattts pv . Outback 2kw  fxr pure sine inverter . fm80 charge controller . victron battery monitor . 24 volts 450 ah surette batterys . off grid  holiday home 
  • WanderingGooseWanderingGoose Posts: 5Registered Users ✭✭
    Another poster pointed out I need another panel to run a 48volt bank. I'm looking to set these 8 up on a 24 volt system.
  • mcgivormcgivor Posts: 2,205Solar Expert ✭✭✭✭✭
    Remember to factor in the self consumption of the inverter 46.5W (18W sleep mode)  this alone would br over 1Kwh if left on 24 hours, nominal efficiency 88% so around 12% loss or more depending on loads. 
    1500W, 6× Schutten 250W Poly panels , Schneider MPPT 60 150 CC, Schneider SW 2524 inverter, 400Ah LFP 24V nominal battery bank 

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