You are right Flux.
Even ignoring the Alternator Efficiency, the Motor Efficiency, cabling resistance, the Mismatch in Power Factor that is often important for a Generator/Motor circuit... The most important aspect of the difference is that the HP Rating are absolutely not the same. Thanks to marketing.
Electric Engines are rated conservatively for continuous duty. 1 HP means that you can use 1 HP 24/7 without issue. Actually, many Electric engine come with an additional SF rating that indicate 1.10, 1.20 or event 1.8. This Safety Factor clearly state that the Electric Motor can, in reality, be use at 1.10 HP, 1.20 HP or even 1.8 HP continuously, 24/7 without any risk of damage. The rating of Industrial Electric motor is often more a question of it peak Electrical Efficiency at a certain load. If an Industrial Electrical Motor is 90% efficiency at 1 HP but 50% at 1.5 HP, it is likely to be rated 1 HP and it will show a SF of 1.5. Obviously, marketing can also play a role here since the same motor can also be rated 1.25 HP but only be 70% efficient. Careful machine designer generally avoid those.
You indicate that some Electric motor will provide 3X the torque. Some industrial motor will even do better than that if you supply enough electric power (at proper Voltage). Actually, it is not rare that an Industrial Electric Motor will have known rating for 2X or 3X the load. These rating are like the one of Inverter. 5 Minutes Overload at 2.2 HP for example -- This is often limited by the Heat Production since the motor might not be that efficient at this load point.
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Stationary Gas Engines are rated by Gross BHP or Marketing BHP. See the power curve of these B&S Engine -- http://www.commercialpower.com/display/router.asp?docid=77788 . The power curve of these "Power Built OHV 5.5HP" and "Power Built OHV 6.5HP" are nearly identical. If you look carefully, the GROSS BHP is effectively about 5.5 and 6.5 BHP. However, the MAXIMUM NET BHP is virtually the same at 2600 RPM or 3600 RPM. Both top at about 3 KW MAXIMUM NET BHP. This clearly shows that the additional Gross BHP can't be converted to Electricity.
So, we see an engine potential of 3.6 KW then realize it can't do better than 3 KW - That, under any circumstance.
Now, this is still MAXIMUM NET BHP. What mean BHP...? Break HP. This means that if you put any gearbox or generator, the efficiency of these apparatus are not accounted for. The rating is therefore lower. Usually, at least 10%.
Finally, no Gas or Diesel engines are rated publicly for 100% duty cycle. (These rating exist but are hardly published by anyone). Usually, if you want the engine to last, kept it load bellow 35% or 40% of it NET HP -- Not BHP (Up to 60% with some Diesel and 85% with JET ENGINE). Past this point, the engine wears fast, in some case, very very fast.
So, if you are charging Battery for 1 Hours at the time (an almost continuous duty). You don't want to use much more than 30% of it NET HP. This is likely about 300 to 500 Watts for your 4HP engine.
Rating of Generator: Don't be fool by the advertising of some/many "low cost" gas generator. Surge Capacity of 5300 Watts, continuous duty of 4000 Watts, Engine B&S of 5.5 HP... It does not mean the machine can do it. It just means that the Electrical Generator might be able to do it if the "Primary Mover" supplies the power. To me, that should be classified as "illegal" advertising - but most company does it.
