I used a 750 watt inverter in a 2004 Xterra during the past week. Rule of thumb (from the manual) says divide the output watts (add up the fan, light, TV, etc) by 10 to get the amp draw from the battery. I was pulling about 400 watts, or 40 amps from the battery that's rated at 45 Amp-Hours. I killed the battery more than once and had to use jumper cables. I understand that automotive batteries are never supposed to be drained that low and it will eventually kill them. By the end of 6 days, I was getting about 45 minutes of AC power before I had to run the engine for an hour to recharge - not a good thing. Used a lot of gas for a little power. I then borrowed a 7500 watt generator and that did the trick, although it was noisy and stinky. Look into "Yellow Top Battery", a deep cycle auto battery with a decent AH rating as long as your alternator can keep up with it.
Rob