minimum on wattage you need to keep one normal deep cycle battery up
That is near impossible to answer as how many watts of panel you need depend on variables that change and may be different for different people
- how many hours of good quality (bright) sun?
- how depleted the battery is at the beginning of the day?
- do you want or need to bring the battery to 100% charge before nightfall?
- how much power (watt-hours or amp-hours) are going to be used during the daytime hours, while the battery still needs recharging?
One way to look at sizing the panels is to first look at the amp-hour capacity of the battery(s). Say one 12 volt, 100 amp-hour battery (rated at the 20 hour rate) Such as
this Trojan... Take 10% of the capacity, 100 amp-hours x 10% = 10. We would be well served by a PV panel or array that put out 10 amps maximum at the voltage appropriate to the battery. That is about as high an amperage to not harm the batteries and high enough to effect as quick a charge as safely possible.
Since the really good sun hours may only be 4 to 5 that (in theory) could supply 40 to 50 amp-hours of charge. In the real world that will fall short for many reasons. But there is also the before and after "good sun" hours that help out. If one is
real lucky
maybe up to 90% of that power produced by the panels will be pushed into the battery.
If you are just running some LED lights you don't need solar recharging unless you take extended trips and always boondock. IMO.
If you have larger power needs, like for running a 12 volt refrigerator or cooler you still might not need any solar, depending on length of time away from grid power or a generator source.
The big thing everything hinges on is how much power is withdrawn from the battery. Note: if the battery has been brought down to less than 70% charge, it is best for the battery life for it to be brought back up to at least 90%, preferably 100%, within a short time (less than 24 hours), otherwise irreversible sulfation may occur.