There's no magic here.. 1000w, regardless if it's a 500watt amp running at 2 channels into 4 ohms, it's drawing 1000 watts, and the power supply will pass at least that much to your battery.. if you run a competition amp that is rated at 125 x2 at 4 ohms into two 4 ohm speakers, you'll burn 250 watts, if you run two 2 ohm speakers, you'll burn 500 watts, if you bridge it into a single 2 ohm speaker you'll burn 1000 watts.. this would have the same relative draw on your battery as the afore mentioned 1000w amp driving 2 4 ohm speakers at 500w. If you want to be more efficent, and get the most from your battery and alternator, look into a D class or E class amp for your sub. They are around 25% more efficent then the standard AB class amps... result: your 1000 watts to the speaker pulls 1100 watts from your battery, rather than 1300 watts.
Bryan