Battery_Power

AH means Amp Hours. (max amount of "saved" current in a battery)
 An ampere-hour electric charge unit = 3600 coulombs (ampere-seconds).
  The commonly seen milliampere-hour (mAh) is equal to 3.6 coulombs.
    1 AH battery can light 1 amp bulb for 1 hour. Simple.
    2 AH battery can light 2 amp bulb for 1 hour (or 1 amp bulb for 2 hours).


the amount of electric charge transferred by a steady current of one ampere for one hour.

Although it is not a direct measure of the energy in a battery (like the joule (J) or watt-hour (Wh)),
  it is a common rating of how long a battery will last
   (for rechargeable battery, how long it will last when fully charged).

Most decent batteries will be from 650 to 800,
  but your needs depend on the accesories in
    the car,
    the climate you live in, and
    the period you are keeping the car.
    The higher the number, the longer you could accidentally leave your lights on ...and still have the car start!

Ampere-hours (A-h) is the product of
    the hrs a battery can deliver a certain amount of current times
    that current_amps, for a
    particular discharge period.

This is one indication of the amount of total energy a battery is able to store and deliver at its rated voltage.

...This rating is rarely stated for automotive batteries....
In car battery, it is not rated as AH as much as Cold Cranking Amps.
As you can imagine, you often crank your engine for few seconds at at time.
So it is more important that car batteries can give a huge jolt of amp ...than long slow drain.
Of course if you are using your car battery to power other devices then AH becomes also important.