As Pale Rider says, regen on an ebike is a bit of a pointless idea. If you can ride along and regenerate as you go, why not dispense with all the gubbins and just have a normal bike? You can't compare it to a car. My Wisper is a heavy old bus, and the one time the battery ran out on mine (error on my part, failed to fully charge it) I had to push it up a steep hill near my home as I simply could not pedal it. 20% hill, 37" bottom gear and 26 kg of bike and battery (not to mention the rider) were an impossible combination. I was very unfit then; I might do better now.
Although it's heavy, it flies along when it is in the mood. The benefit for me is on the hills. On the flat, I am pedalling at over the 15 mph cut-off, so I'm not getting any assistance. On the hills, it's like a giant hand on the back of your saddle, just helping you along. I would estimate that a hill that I can climb at 4-5 mph unassisted, I ascend at 8-9 mph on the ebike. It's still work, but for me it made the difference between cycling and not cycling. Commuting was going to be my route back to health and fitness, but with a hilly 13 miles each way, it was not possible for me on a normal bike. After a year on the ebike, I had lost weight and gained fitness, and now I am planning on switching to my normal bike full-time. I have rediscovered the pure joy of getting out on a bike and covering the miles. I don't think it's exaggerating to say that the ebike saved my life. Without it, I would still be sitting on the sofa, 5 stone overweight, pre-diabetic, wondering if I would ever be able to bike to work ever again. It's not a pushbike, but it certainly isn't cheating either. Horses for courses.