Well, since you need at least a 20 amp power supply to run a 100 watt
radio, you'd need 20 amps at 12 volts of batteries.  Now if you'd
think rather than complain about sarcastic replies, you'd realize
that unless you just graduated basic training marching 20 miles with
an 80 pound backpack, this isn't a particularly good idea.  It's not
amp-hours that counts, you have to have the available current  for
the instantaneous peeks.
A car can be more efficient, because to carry a hundred pounds of
batteries isn't a problem for a car.  It however is a problem for you.