If your starting battery is going dead in about a month while you vehicle is in storage, it would be fair to assume you are using about 60-75 amps(typical RV sized starting battery) in that period. To break it down further, we can assume 2-3 amps per day.

In order to replace 2 amps per day, you will need a minimum of a 5 watt panel, larger if my guesstimate is off. Have a look at our 5-10 watt solar panels.

The first step is to figure out how much of a drain you have. Short of having some expensive testing equipment, you can get a fairly accurate estimate by first knowing how large your battery is in storage ability. As stated previously, you can figure 75 amps as a safe bet.

If your battery takes 3 weeks to discharge, simply divide that 75amps by 21 days. Now you have your estimated drain per day. In this case, about 3 amps per day. All you need know is to select a panel that will produce 3 amps per day. A 5 watt solar panel will produce about 1/3rd of an amp per hour of good direct sunlight. So, in 7 hours of sunlight, a 5 watt panel will make about 2 amps per day.

Am I saying you need a 5 watt panel? No, this is just to illustrate how to figure out what size to use. I can say that the smaller 2-3 watt panels are typically right for maintaining a car in storage, and 5-10 watts is usually enough for a parked RV.

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