So 30 Ah alone doesn’t really tell us everything we need, as we need to know the voltage but let’s assume 12v. So your 30Ah battery has 30×12=360Wh capacity.
Well, imagine you had a 100w solar panel in full sunlight at the optimum angle, assuming no large inefficiencies in your solar controller or wiring then 3.6 hours would see a fully charged battery. So a 50w panel might suffice?
Of course that’s laboratory conditons. In the real world your panel/s will not always be optimally positioned, the sun won’t always shine and your charging equipment will not be 100% efficient.
I have 600w of panels on my motorhome and a 220ah 24v battery (5280Wh). Its winter where I am currently in Southern Spain. We are getting 6 hours of good sunlight per day, but at a low angle (my panels are flat on the van’s roof) and I figure I’m generating around 100-200W for 6 hours so that’s 600-1200 Wh per day which is only charging my battery around 20%.
So for a 360Wh battery I’d go for 100w panel.