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Old 12-07-2010, 09:01 PM   #21
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Solarblvd.com is selling Evergreen 200W 12V panels for $399, down from $900 - you could do the entire project using this panel for under $700 plus labor.
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Old 12-08-2010, 01:01 AM   #22
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Originally Posted by JBinKC View Post
I have a very clear picture of what I want: two charged batteries each day so I can run my onboard propane heater and have sufficient lights for cool, even cold, nights in national parks. That said, an expert told me that two 100-watt panels and the requisite accessories would probably fill the bill.
200 watts of solar panel, if you are parked in bright sun all day, will give you a maximum of about 1 kWh. Since they will be on your roof and not pointed directly at the sun, and probably dusty, figure 3/4 of that or 750 watthours. Assuming you have big enough batteries that are wired properly and fully charged when you start, 750 watthours in yields about 75% of that or maybe 600 watthours useful output. LED lights all night, maybe 5 or 10 watts maximum, no problem. The propane heaters draw about 3 amps times 12 volts or 36 watts. Add the 10 watts or so that your refrigerator controller will use, and maybe 10 watts for everything else. Assume your heater runs 50% of the time. House loads, 20 watts, heater blower 18 watts (at half time), lights 10 watts, total 48 watts, into 600 watthours is about 12 hours run time. This is about what we have seen, *if* everything is in good shape. More solar and more batteries are a good idea. $2,000 for 200 watts is a bit steep but maybe not if it includes competent labor.

Battery maintenance is critical. A typical Airstream trailer battery is a single type 27, which is rated at about 85 amp-hours times 12 volts is about 1000 watt-hours, but you don't want to discharge it more than 50%, ever. And at 0 degrees F capacity is already reduced to about 75%. You want at least double this amount -- I use T105 golf cart batteries in marine battery boxes -- and preferably more, otherwise you (a) will not be able to hold all the energy your solar panels produce and (b) you'll damage the batteries by draining them too far. Your two batteries (either two 6-volt in series or two 12-volt in parallel) will probably be enough if you are careful with use and maintenance. "Maintenance free" or gel batteries don't need watering, but they are much more critical about charging. Be sure you have a good charge controller, preferably a MPPT type, to get the most out of your solar panels. And watch the voltage -- if it gets below 10.4 or so, turn everything off and wait for the sun to come up or you'll use up one of the batteries' nine lives... fortunately this is less critical if the batteries are really cold, but 10 volts should be the absolute minimum even then.
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