If the battery was charged then the only draw on the altenator would be the rest of the engine electronics (coil, ECU, etc.). David's estimate is probably pretty close.
Want to get closer? Figure out how much amperage the engine's electronics are drawing, figure a 60% efficiency in the altenator, and convert the amps to watts (13.4v x amps) and then convert the watts to hp (Watts x 0.00134).
Doing it backwards: 1hp = 746 watts, 746 watts -> 55.7 amps.
We've ignored the efficiency of the altenator, but I'd say that 1hp is pretty close and it probably wouldn't be more than 2 hp.
Pure SWAG!
