I don't think one 60 watt will be worthwhile. We have two and one more is in the future. The angle of the suns rays determine the effectiveness of the panels, for about 3 hours, when the sun is high we get a high level of charge. If the panels can by moved, to follow the sun, they will be a bit more useful. That is not practical for us. In ten years we have never experienced a dead battery.
When our digital volt meter reads 12.2 volts we start the generator or utilize another method of charging. 12.2 volts is about one half of the battery power. When the batteries are allowed to get below halfway capacity, their useful life is shortened.