This gives you an idea of how much:
Mopar Engines - Power Vs. Luxury - Mopar Muscle Magazine
(found originally by t vargo)
The hot rod article says 3 to 4.5hp depending on how fast you spin the very small 60 amp alternator they used. Its reasonable to assume the far larger 200 amp AD244 I am going to use will take even more engine power to free spin.
Going to run a test that might answer this question. A small scale test before a larger build.
For the test engine I have my brand new 5.5hp Honda horizontal shaft engine I bought to make a portable air compressor out of.
The alternator will be an AD244 with the voltage regulator replaced with a big rheostat.
The idea is to take the engine, run it at various no load speeds to see how long a set amount of gas runs for. Then put the alternator with no load on the engine and see how long it takes to burn through the same amount of gas.
We will assume 0.5lb/hr per horsepower.
Then redo some of the tests with a load. Double check some to ensure consistency.
The main objective is to calculate horsepower to turn the alternator at max output for that engine with various pulley sizes (2.6'' on the alternator and 4 and 5 inch on the engine), that should give me about a 1 to 1.5 ratio and 1 to 2 ratio. The max power tests will likely need to do the loaded test first and then run the engine that speed with the alternator running no load after. Max power tests should be run at about 5,000 alternator RPM and about 6,800 alternator RPMs.
Then later on scale it up to see what it would take to run this alternator full field output at target speed, some where around 6,000 or 7,000 RPM.