But the second law of thermodynamics means that you can't get as much out of regenerative braking as you put in to the cars potential energy. It's not going to magically allow your car to recharge itself from braking; all it does is recapture a fraction of the energy you've spent accelerating the car or hauling it up a hill.
Regenerative braking allows you to more efficiently drive within a city.
But the big win is driving at city speeds. I had explained this the other day, but power usage depends approx. on the square of your velocity.
So averaging 30mph in the city would take about 1/4 as much power as averaging 60mph on the highway, all other things being equal.
Normally all other things are not equal, as idling at a city stoplight wastes energy in a gas-powered machine, and pressing the brakes wastes energy in a gas-powered machine.
With an EV idling uses very little energy, and you can reclaim some energy from braking. Braking is still a net loss, but not as much.
Combine the two effects with a 4x power efficiency and it's not at all impossible for a hybrid to get better mileage in city, especially in stop-and-go traffic where the average speed is even lower.
I have a friend who owns a Tesla Model S who says the regen is not as efficient in stop and go traffic as lighter vehicles. The Prius and Leaf are thousands of pounds lighter. (He still loves his car.)
He says the Tesla seems more efficient on county highways than side roads because the losses from accelerating the vehicle's over 4500 pound weight exceed losses from air resistance at those speeds.
