Want to get a small solar panel to keep my propane generator starting battery (Automotive Diehard Gold) charged. Want to avoid having to run the generator for an hour each month just to keep it charged. Can't charge it with an automotive charger because my beloved German Shepherd dog will definitely chew the power cord. On eBay I've seen small panels for doing that ranging anywhere from 1.5 watts up to 20 watts. Have a hard time believing that a 1-1/2 watt panel could keep it charged. Don't want a charge controller or more panel than necessary to do it. But I really really don't want the battery to die from undercharging sulfation. How many panel watts do I need to maintain it?