You can think about it like this
the coltage is 2 v
3000Ah means that if the current is 3000A the battery will provide electrical energy for 1 hour
so simply the formula is like this : E= 2 x 3000 = 6000 WH = 6 KWH energy required
P.S : this calculations are for IDEAL enviroment, but could be applied 99% of times,