How long will a 12volt 7ah battery last with a 24 watt bulb?

-- The power being used is (voltage) x (current).

-- In order to get 100 watts out of a 12-volt battery, the battery has to

produce 81/3 Amps of current, because (12) x (81/3) = 100.

-- The battery holds enough juice so that (current) x (length of time) = 7 Amp-hour.

-- If you pull 81/3 Amps from that battery, the math says that it'll be empty

in 0.84 hour (50 minutes), because (81/3 Amps) x (0.84 hour) = 7 Amp-hour.

-- The truth is that if you discharge a battery that fast (all of its capacity in

less than 1 hour), it won't match the math. It'll begin to sag, groan, and gasp

in less time than you'd expect.

-- 0.84 hour is about 50 minutes. you should not expect 100 watts out of

this battery for much more than 35 or 40 minutes.