it depends on volume. the most powerful device is amplifier and subwoofer. dvd monitor needs less. imho 3-4 hours for battery isn`t problem
you needn`t it. it`s choice for sound pressure competitions, when you need huge amount of energy for big "BOOM"Finally, adding a capacitor as a power buffer might reduce ignition noise, and may improve the dynamic response of the audio system; it won't extend run time. Capacitors are "lossy" (self-discharge). Adding one will decrease your run time.