Most car batteries put out roughly 11.5-13V. That is what you will get when your car is off. When you crank your car your alternator will start producing power. Most alternators produce about 13.5-14.5V.
Most amps made today are rated at 13.8V or 14.4V. Some are rated at 12V, those rated at 12V will generally produce more power at 14V than they are rated for at 12V.
Ex: an amp is rated at 1000wrms @12v in a 1ohm load...this amp may produce as much as 1200w at 14.4V.
yup! a regulated amp will do 1200 watts at 11.5 volts and 1200 watts at 14.4 volts. They draw a ton of current from your electrical system because no matter what the power your alternator is putting out they always pull the amount they need to produce that 1200 watts.