Re: 100W solar panel how much aH lead acid battery capacity require?
You probably want to buy a 12V battery which is available locally. A typical type is a marine battery. It's designed for deep-cycle use, and has thicker lead plates than an automobile battery. (An automobile battery has thin plates suitable to provide high current flow, and then recharge quickly.) A boating shop should carry marine batteries.
Operating day to day you'll find the amount of Amp-hours you can use in an evening, is about the same as what you received during a day's charge session.
On a good day I estimate you'll get 5 or 6 Amperes charging a 12V marine battery. (This is based on testing with my 64W PV panels, pushing 3.5A into a 12V battery.)
Suppose a day's charging session lasts 7 hours. You can calculate Amp-hours as 7 x 6 = 42. It's enough electricity to power lighting or a DC well pump, for a length of time which depends on your total load.