# of watts solar system needed to MAINTAIN coach batteries?

Unless I'm missing something (very possible), all you need to is to offset the parasitic drain on the batteries, which is normally less than 100mW (mine is only about 50mW), plus the self-discharge (assume it's 5% per month for a lead-acid battery). That assumes everything in the coach is off and the only load is for the CO/propane detector, the clock battery, and things like that. 100mW x 24 hrs/day = 2.4 watt-hours/day. With a 70-amp-hour battery, assume a 5%/month self-discharge rate = 3.5 amp-hours/month, or roughly 40 watts/month, or 1.3 watt-hours per day. The total is around 3.7 watt-hours/day. Even at 50% efficiency, a 10-watt solar panel with an output for a couple hours per day would provide 10 watt-hours/day, which is way more than you need. That's why most commercial solar battery maintainers are all in the range of a few watts and work just fine.

Or, the easiest way is to do like other people suggest and buy something with excess capacity and then you don't have to worry about it.

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