KiloWatts (KW) and Horsepower are similar measures, in fact, overseas, instead of horsepower car engines are rated in KW.
http://www.processassociates.com/cgi-bin/convert.exe
1 KW = 1.34 HP - if things were 100% efficient.
a 200A, 12V alternator = 2.4 KW, so it would cost you maybe 3.5-4 HP.