You'd have to look at the birthsheet of the amp. If it's underrated and says that it actually puts out 3000 watts at 1 ohm, and it's tested at 13 volts then you might get close to 2500 watts at 1.5 ohms at 13.8 volts. Plus since so many amps put out the same wattage no matter what their voltage is, its entirely possible to put out 2500 watts at 13.8 volts. It'd be bad for your car though, you'd be putting out more wattage than you're charging. It shouldn't put out more than like 1000 watts constant though. Unless you're always cranking it to some serious rap music with the bass on max.