Sub Question

There is no alternator. The 12V bus on the Prius is supplied by a DC to DC converter from the HV battery. Normally the average power draw from an audio amplifier is well below its rated output. Unless you are using a freakish amount of bass, the DC to DC converter should be able to keep up.

Obviously the energy used by the audio system comes at the expense of mileage. 1000 W is about 1.34 hp, so you can see that the mileage hit is real but not huge. I can't give you a mileage number without having the driving conditions for the Prius. For example, if you listen to music while standing still, the mileage hit is infinite. On the other hand, at highway speed it is fairly modest.

Tom

 

All the power in a Prius comes from the gas tank, so the answer is yes mpg is affected. How much depends on how much power your audio system uses. The maximum power available from the Prius DC/DC converter which charges the 12 volt battery is 100 Amps times 12 volts = about 1200 watts. So you need to determine if your system draws more current than is available. I wouldn't want to be drawing maximum power on a continuous basis either. The 12 volt battery is quite small and you certainly couldn't run your system without the car in Ready mode. That would mean a dead battery in seconds.