Solar for boondocking

System

I am looking at installing a solar panel on my 16' TT. I only have one 12V battery, and will only be using DC for water pump, and lights at night. Will only be out 2 to 3 days. What watt panel would I need to maintain a charge on the battery?

Cariboocoot

Re: Solar for boondocking

Welcome to the forum.

In order to give you advice about what size panel you need, we'd need to know what size (in Amp hours) battery you have. It would also be helpful to know what your expected power use is (in Watt hours or Amp hours) and whether you're looking for enough panel to off-set this use, completely recharge the battery, or just maintain it when there's no loads.

The basic formula looks like this:

Amp hours of battery * charge rate (5%-13% usually) = desired current.
Desired current * charge Voltage (usually 14.2 on a "12 Volt" system) / efficiency rating of panel(s) & controller (typically 77%) = total Wattage of array.

Often the battery will be 100 Amp hours, so it would look like this:
5 Amps (minimum) * 14.2 Volts/0.77 = 92 Watts

RV's/campers tend to be at a disadvantage because they usually have less than ideal insolation due to being parked as needed rather than in the best location for solar. Some have devised "portable" panel set-ups which allow for best sun exposure on the panels while the camper stays in the shade.

System2

Re: Solar for boondocking

Will this help in deciding? SRM-24 is my battery.

http://www.interstatebatteries.com/cs_eStore/content/product_info/marine_f.asp

Cariboocoot

Re: Solar for boondocking

Well there's the first problem. These RV/Marine batteries aren't really suitable for deep cycle use, despite the "deep cycle" label. The specs given don't have the "standard" "20 hour rate" we normally use for RE purposes, but it looks like it's about 70 Amp hours. Another issue is the Interstate batteries themselves haven't got the best reputation for RE applications. Unfortunately I know this first-hand.

But based on 70 Amp hours, for full charging you'd be looking at math like this:

10% charge rate: 7 Amps @ 14.2 Volts (charging Voltage for a "12 Volt" system) = 99 Watts, less typical 77% efficiency derating = 129 watts.

That is still only an approximation. How much sun actually falls on the panel will make a difference, as well as how much power you actually draw out of the battery between (and during) recharge. Those are two big, important variables. What happens if it rains while you're camping? Do you have a back-up power source such as a small generator?

If so, you can possible reduce the size of the panel to the minimum 5% charge rate. Otherwise the "rule-of-thumb" for camping is as much PV as you can afford/fit on the camper, because it's easy to have too little and practically impossible to have too much. :roll:

Frankly the Kyocera 135 Watt panel is a readily available bargain in power: http://www.solar-electric.com/kyso130wa12v.html Combined with a small charge controller like this: http://www.solar-electric.com/ss-10.html it could work for up to 100 Amp hours fairly well. Not a specific recommendation; just for example.