200 W of Solar

Depends what you want to do for mounting (portable vs roof) and how many amphours you are taking out of you batteries daily. Get a voltmeter and starting with a fully charged battery, use power normally until the voltage (with everything off for half an hour or so) is around 12 or 12.1 volts. That is about 50% state of charge or roughly 100 amphours out of your golf cart batteries.

Divide by the number of days (and partial days) you've been camping. That is the number of amp hours per day you're using.

Next, multiply your daily amp hours by 4.5. That is roughly how many watts of solar you'll need to maintain battery charge under average 3 season conditions with an unshaded solar system. Many folks find 100 watts is sufficient, others need more, it depends on what you are running in the trailer.

Renogy stuff works fine but it's not exactly inexpensive. If you do decide you need a 200+ watt portable solar system, the most bang for the buck is to get a standard 60 cell residential (250-300 watts) module. These are often available left over from installations for 50 cents a watt or less. Try CL.

You will also need an MPPT charge controller, 30-40 amp, which you can get on the 'zon pretty cheap now. Some 10 gauge cable, a fuse and fuse holder, and back leg materials to tilt the module and fold up when not in use and that's it.

If you don't want to lug around a few hundred watts of PV and want to roof mount it instead, consider getting an 72 cell commercial module (350-400 watts) and roof mounting that. Those modules are the standard for commercial solar installations so are the least expensive per watt. The length of those big modules is almost the same as the width of the narrow rpods so if you get the right one it will extend side to side so you don't have to put holes in the rpod roof.

If you run two modules in series then both of them need to be completely unshaded. All the cells in both will be series connected so shading any cell will take out both modules.

If you run two modules in parallel then shading on one doesn't effect the other. However, since MPPT controllers step the voltage down, you don't have enough voltage on nominal 12v modules (36 cells) to use one.

Thats why I prefer a PWM controller for portable applications subject to shading with 12v modules. Cheaper too. If you go with the 60 cell or higher modules then you need to use an MPPT.

Not sure what the convenience is with the Renogy kit? You can get a Renogy controller if you like that and still get a residential 60 cell module as the power source, and be ahead a couple hundred dollars with more power.

craigslist. You could also try EBay but watch the shipping costs.

You should last several days if you're just using lights and the water pump. The pump has a very low duty cycle (% on time) and the lights are LED so very efficient. Just turn off the lights if you're not actually using that area. The fans (furnace and bathroom vent) use more if the weather gets hot or cold. I'd say 200 watts solar would probably be overkill in your situation. No harm in it though, the controller will prevent battery overcharging.

Maybe not the furnace, that fan uses a lot. Especially in the late fall when it's both cold and solar irradiance is low. Spring is better because we have longer sunnier days.


But do consider what your options are if you run into a string of cloudy days, as we frequently do in the East and Midwest, it doesn't matter how much solar you have, it will be producing very little, about 10% if it's heavily overcast.

For confidence you wont run out of power I'd suggest you consider a dual battery configuration to get through some cloudy days, and a small generator for backup. You can run the genny from your propane if you don't want to carry gasoline. Nice and clean that way.

Not lame, that's a good question.

If your current battery is a year or more old now (sounds like it might be) then I'd go with two new 6v golf cart batteries in series. Case size GC2. Very common battery in the golf cart world so can be had for a good price. Those are the largest size that will fit, are true deep cycle so can tolerate deep discharging, and everything else being equal series is better because both batteries always get the same charge and discharge current. When they're in parallel they don't, one will always have a little lower internal resistance than the other so will take and provide more current.

If OTOH your existing 12V battery is near new then it makes better economic sense to keep it if you can buy another battery same model and same manufacturer. If you do that connect them in parallel but pull your positive take off from one battery and your negative from the other, that will help keep them balanced.

One other word of warning, dual batteries are going to add about 60+ lbs to your tongue weight, be sure your tow vehicle can handle it.