Amplifier power draw
At 100% efficiency, the power drawn from the battery will be exactly the same as the power output to the speaker.

Real amplifiers are never 100% efficient, but a class D amplifier can be pretty efficient.  TI claims > 88% for the chip used in the amplifier you are looking at.

You should really look at the tas5630 datasheet.

Here is power wasted vs. power output.  It is nearly a straight line.  Notice a few watts are drawn all the time, even with zero output:



Another graph to study carefully is this one.  Maybe two car batteries in series (for 24V) is not enough voltage to get your desired power output: