These are two different entities. Its like asking how much water will flow with pressure. With the following formula W = Amps x Volts you can see the relationship they have with each other, the end product resulting in Watts.
A volt is a unit of electrical potential, joules per coulomb. An ampere is a unit of electrical current, coulombs per second. The two are not related, unless you also know the resistance, in which...
Use an ammeter connected in series with the supply.Make sure the ammeter is of sufficient size to cope with the maximum current.Add up all the wattages of devices used (stamped on them) to get a clue...
Not all 12 volt batteries actually use acid. Lead/acid storage batteries do, however, and in those the acid is generally 4-5 molar sulfuric acid. "How much" acid depends on the volume of the battery...
DC to AC?? Invertor boxes are used in Autos. you need to know what the load of the device you are using to buy one that will work. Simply plug it in the cig lighter and there it is.