I suppose one could say that, scientifically speaking, 1 horsepower is equal to 746 watts but there is more to consider, such as but not limited to, elevation above sea level. (deduct 3% for every 1,000 foot.) So, if we take your generators maximum output ratings of 24 vdc nominal and 125 amps we get 3,000 watts. Another consideration is, will this generator run at maximum output without self destructing in a matter of time?
Now that we have the wattage it is simply a matter of dividing 3,000 by 746 and that gives us 4.0 hp. Here is another consideration, will the engine run at maximum hp without self destructing in a matter of time? Other considerations may be engine rpm as related to the generators actual output voltage. How the generators power to the battery bank will be controlled? Pulley ratios or direct drive?
Personally, I wouldn't consider anything less than 6.5 hp right from the start. Call it "gut" feeling. Besides, I don't want to take all of the fun out it.
There are lots of people out there on the internet sharing their experiences with projects more or less similar to yours. Here are a few just to get you started.
http://www.fieldlines.com/story/2009/9/4/0811/74145http://www.theepicenter.com/tow02077.htmlhttp://www.ibiblio.org/kuphaldt/socratic/output/dcgen.pdfI really don't know what the fascination is with converting "lawnmowers" to "generators." Something portable I guess. Not that I think that this is what your doing. It sounds to me like you want something more advanced, what with choosing bio-diesel and all.
