The electrical drag will just be your voltage x current draw / efficiency, so a 12.8V battery at 50 Amps with a 60% efficient alternator will take 1.07kW. 50 amps was just a guess but you could measure your amp draw if you want an accurate measurement.
According to
Wikipedia "Very large automotive alternators used on buses, heavy equipments or emergency vehicles may produce 300 amperes. Very old automobiles with minimal lighting and electronic devices may have only a 30 ampere alternator. Typical passenger car and light truck alternators are rated around 70 amperes, though higher ratings are becoming more common."
So it looks like my 50 A guess was pretty close, somewhere around 1hp (38A) - 2hp (70A).