How much power does my car subwoofer actually need?

Match the amplifier's RMS output to the sub's RMS rating, not its peak/max rating. The big "watt" numbers in product titles are usually peak figures and are largely marketing. A sub fed clean, adequate RMS power will sound better and last longer than one starved or overdriven by a mismatched amp. Always verify the RMS spec on the manufacturer's documentation before buying an amp.