1: How much energy can your battery store?
You need to know how much energy your battery can store and then select a Solar panel that can replenish your ‘stock’ of energy in the battery.
Battery capacity is measured in Amp Hours (eg 100AH). You need to convert this to Watt Hours by multiplying the AH figure by the battery voltage (eg 12V).
For a 100AH, 12V battery the Watt Hours figure is 100 x 12 =1200WH
(But realistically, you can only use 600WH, sticking with the 50% rule)
Using this rule means the battery could supply 600W for 1 hour, or 300W for 2 hours i.e. the more energy you take, the faster the battery discharges.