I am going to be building a solar USB charger for my iPad soon and want it to have a built in battery for the dark part of the day. I know that if I let it die it takes about 8 hours to charge the iPad with the 2A 5v charger.

So I guess I need a 8Ah battery to charge it and a solar panel that can recharge the battery in a 12 hour day.

The problem for me comes with the solar panel size. I already have the 5v regulators from a previous project but the panel and battery I do not.

Am I correct with the size of battery and what size panel should i get?

 

A "stock" iPad charger delivers 10W as 2A at 5V. The latest battery is over 40 W•hrs, so it takes over 4 hours to charge, and longer if you're using it at the same time.

The key to your plan is determining how much you'll use the iPad (consumption) versus the real output of your panel (production). Perhaps you have 12 hr days, but you don't likely have 12 hrs of light equivalent to high noon.

I'd look for a 10-12V panel rated to at least 10W. This will ensure you'll overcome the various losses (blocking diode, regulator) and still get some charging at less-than-full sunlight. You can back off on the size if you have a lot of sun each day and little daily usage of the iPad, but that seems unlikely.