California has bizarre regulations regarding range extenders.
I also don’t see why 50 hp is a good target. The oldest Model S cars can drive on the freeway (at moderate speed) using maybe 25 kW (33 hp). So a 25 kW generator would allow indefinite freeway driving at moderate speed. But almost no one does this except maybe long haul trucks that trade drivers.
IMO the right way to think of it is: a 25 kW generator will almost fully recharge the battery in under 4 hours. If you drive uphill or fast for two hours, and you run that generator, you have an extra 50 kWh. If you want to drive 10 hours (shudder), that’s an extra 250kWh — you should avoided about three long charging stops, so maybe one actual level 3 stop gets you there even if you drive moderately fast.
And you can stop for the night (or sightseeing or whatever, as long as you park outdoors), and you’ll be fully recharged afterwards. I would appreciate a 5kW onboard generator for this purpose!
Tiny engines (ie. sub 20 horsepower) have pretty poor efficiency, and tend not to meet modern emissions requirements (since they haven't been developed with automotive use in mind).
Nobody is putting much R&D into new engine designs.
Lots of countries have laws saying an engine in a car can't be running without a driver present.
For all those reasons, tiny range extenders on large batteries don't tend to exist.
Instead you get moderate or large range extenders paired with smallish batteries (ie. total range 50 miles). And they still have trouble if you drive fast, heavily laden, up a hill, on a hot day for more than the battery capacity.
I also don’t see why 50 hp is a good target. The oldest Model S cars can drive on the freeway (at moderate speed) using maybe 25 kW (33 hp). So a 25 kW generator would allow indefinite freeway driving at moderate speed. But almost no one does this except maybe long haul trucks that trade drivers.
IMO the right way to think of it is: a 25 kW generator will almost fully recharge the battery in under 4 hours. If you drive uphill or fast for two hours, and you run that generator, you have an extra 50 kWh. If you want to drive 10 hours (shudder), that’s an extra 250kWh — you should avoided about three long charging stops, so maybe one actual level 3 stop gets you there even if you drive moderately fast.
And you can stop for the night (or sightseeing or whatever, as long as you park outdoors), and you’ll be fully recharged afterwards. I would appreciate a 5kW onboard generator for this purpose!