xkcd

You really don't need 90hp. Coasting on the freeway takes less than 10hp, depending on how big of a block you drive, so as long as the average is around that, the generator can keep the battery charged forever, and the battery handles any surge in power you need. It's only a problem if you drive like a jerk, and floor it out of every light or speed down the highway at 100+mph, and do it long enough to drain the battery.

But the brilliant part is that you can design the generator motor for single, constant RPM. I can't emphasize how much easier and more efficient that makes everything, vs. having to engineer a huge power/rpm range that can handle a dynamic load.