There is not a direct 1/1 correlation between stereo output watts (RMS) and the wattage draw of the stereo (VA or watts, or amps). So because a stereo is advertised as 400 watts, that doesn't mean it will draw that much, even if fully cranked. While a large stereo will indeed kill a small car battery quite quickly (a Prius battery even quicker since it is so small) as has been stated above, it will make almost no calculable difference in mileage. Icarus
