10 watts or less
The 5 watt panels that go on the dash and plug into a cigarette lighter are enough to keep a full battery charged. My truck battery lasted 10 years with one of those. For two batteries, you would want 10 watts. Everything I've read says 10 watts or less doesn't need a charge controller. It doesn't have enough amperage to overcharge a battery. Parasitic drains on my trailer run about 5 watts, so a 5 watt panel would not be enough to keep it charged if hooked up to a trailer, because the sun doesn't shine 24 hours. A 5 or 10 watt panel with no charge controller, and with battery disconnected from the trailer would do the trick. It won't necessarily charge your battery, but it will keep it from draining.
I would recommend a larger solar panel on the roof, with a charge controller. It tops off your battery during the day. Change your light bulbs to LED, and you won't need that much power. Some people are power hogs, but we get by fine with 100 watt panel and two batteries. The only time I had trouble was up in Montana when it rained a few days in a row. I put a smaller portable panel to catch the morning sun, and we did fine.
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2009 Roo 21ss + 2007 Superduty 6.0
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