sizing a PV for an RV...

Hi there venerable knowledge laden ones,
I'm trying to set up a small PV system for my RV around Bozeman, MT.
My conservative estimate for daily power gulp is between 1100 watt/hrs, and 1500 watt/hrs. What I've found is that there is a whole lot more info avaliable out there on this thing than I have hours in the day to wade through, so I have to appeal to the experts.
I got some advice to try 300w of modules, a small 10 amp charge controller, small inverter (_____), & a battery bank of 4 90-100 A.H AGM's.

Now my main questions are:

-will I have to completely rewire my trailer, which is old and I believe is wired for 12v (a plunge outlet for external power rated at 35 amps, 120v)?

-how complicated/beneficial is it to run a fan and lights (main draws) off of DC, rather than AC

-Does this seem like enough juice for ~1kwh?

any advice sarcastic or sincere is appreciated immensly.


  • System2
    System2 Posts: 6,290 admin
    Re: sizing a PV for an RV...

    From a boaters perspective, there are two ways to come at this.

    1. generate more juice
    2. use less juice.

    So for example, my navigation lights burn one amp, each. I have two of them, bow and stern. They have to run 8-10 hours a day. CRIKEY, that's 16-20 amps a day for two lousy little lights that hopefully nobody will ever see!!! But I replaced those bad boys with LED lights which burn 0.1 amp and oh my. How much solar would it take to generate 20 amps in a day? Easily, like 80+ watts, which would cost probably $450. Those two $40 led lights are lifesavers.

    So you might not want to use a whole lot of LED lights in your trailer, but the principle holds. Are there 12 volt thingamabobs which you can buy for only a few dollars that are a lot more efficient than the stuff you've got in there, now? There are definitely 12volt LED area lights that don't cost a lot and they're a LOT...LOT more efficient than a 75 watt light bulb.

    Also, one other principle. If you you "12volt" (really 16 - 17 volt) panels to charge your 12 volt batteries, then use an inverter to generate 110 volt juice from those 12 volt batteries, you're wasting a lot of energy in the inverter. Would it be more efficient to just use 12 volt "stuff" around the trailer as much as possible, and only use the 110 for stuff where you have no option, like your laptop computer? I think so.

    Also, AGM's sure are nice, but I don't think they charge THAT much faster than gels, and they cost a lot more iffin' I remember rightly.

    Just my dos centavos.
  • mike95490
    mike95490 Solar Expert Posts: 9,583 ✭✭✭✭✭
    Re: sizing a PV for an RV...

    12V wires are for 12V, if you want 120V, you need to string 120V household romex around.

    Use flooded batteries for the first set, and learn to treat them well, when they die, replace with AGM style, and reset/calibrate your charger to AGM.

    You can get 12V DC fluorescent lights & 12V muffin fans

    1,000 WH of load needs:
    5 hours sun &
    apx 300W PV panels
    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: ,

  • System2
    System2 Posts: 6,290 admin
    Re: sizing a PV for an RV...

    i am so pleased to be able to share some of the knowledge those aforementioned experts have bestowed to me.

    being realistic i really am only kind of competent on answering one part of your questions but i'll give a stab and it and we'll see how competent i come out smelling =)

    I am gotten the sense that its always wiser to focus on reducing your consumption as a pose to worrying about generating more energy, and in regards to the lights i couldnt agree more. As far as the conversion of DC to AC i'd again have to concur that the efficiency of the inverter should be taken into account however, its my understanding that the lower the draw on the inverter (below its rated output) the more efficient it becomes, so if an inverter is only pushing 10% of its rated output it might be closer to 95% efficient then at 50% draw which might be 85% efficient (these are just rough numbers for the example) Fans and Lights are perfect to run off DC, and if your rig is currently wired for it then its more then likley very cost efficient to stick with it. I would however consider (if it already has an old inverter in it for your laptop or tv / whatnot) swapping that out for a new one.

    I'm not sure how big your RV is but if its got accomodations such as a TV or air conditioning then i would consider also weighing the costs of replacing these with newer models that are more efficient as a pose to upsizing your solar system.

    in the case of total DC to AC conversion i've found that only in larger applications such as homes is it really worth it to convert and even then mostly for the convenience factor and the cost effectiveness of readily available AC appliances and lighting etc.

    Some thoughts i hope have been helpful and not mis informed (im by no means an expert)
  • niel
    niel Solar Expert Posts: 10,300 ✭✭✭✭
    Re: sizing a PV for an RV...

    if you use an inverter it takes energy to invert 12vdc to 120vac so a portion of the power will be wasted in doing that. also, know that an inverter on, but without loads to it still draws a minimum amount of power.
    as to the system proposed i say the battery capacity seems ok, but you will need more pvs to charge them as you must replace what you use. i believe they went by a 5hr full sun per day thing and just took your max 1500kwh and divided buy 5hrs for 300w needed from pvs. that 5 full sun hours may be good for sunny summer days, but you get trouble on bad weather days and other seasons that have less sun intensity and sun time available. furthermore, you have to aim the pvs to the sun and if you leave them flat on the rv you will have more losses.
    to top it all off, the stc ratings aren't realistic as it is rare anybody will get the stc ratings of their pvs with ptc or cec ratings being better indicators of the wattage available. i'll add that you could gain back some of the charging inefficiencies too by getting a good mppt charge controller with a battery temperature sensor. you may not always get to power all you want to and 1100-1500wh per day is excellent as i doubt you'll conserve much more than you are already, but i think roughly 400w stc in pvs into an mppt controller with about 400ah of batteries should start you out very well. if you'll need more pv then you can add to it later so give yourself some leadway in your controller selection and other components of the system for their expandibility.
    as to your old wiring i can't say to much on that without seeing it, but any new fans or lights added should get new wire with fuses or circuit breakers all sized appropriately.