In the typical Model S, in daily practice. Wikipedia says that 80% would be theoretically possible. As regards electric cars, the DoE explains that the efficiency between the electric motor and the wheels amounts to 60%. Let us consider the electric motor efficiency {\displaystyle \eta _{eng}=0.6} and the braking proportion in towns {\displaystyle p=0.46}and on motorways {\displaystyle p=0.1}. Let us introduce {\displaystyle \eta _{recup}} which is the recuperated proportion of braking energy. Theoretically, it can reach up to 80%.

But after you recover that kinetic energy, you have to convert it back to kinetic energy again to make the car go. Coasting is good. Braking is evil. I'm hoping that the less aggressive TACC braking of 8.0 over 7.1 will give some reduction in energy consumption.

Below 40%, for sure, this is "full round-trip from battery back to battery" considered. There are scientific papers on this.

Here's a rough calculation based on the info presented on my LEAF's dashboard. It says that if I slow down using regenerative braking from 65 mph to 25 mph, turn the corner, I can then accelerate to about 50 mph using the recaptured energy. From high school physics: KineticEnergy = 1/2 Mass Velocity^2; The car doesn't change mass, so you can compare the energy by just comparing the Velocity^2. Before Slowing Down: energy = 65*65 = 4225 While Turning: energy = 25*25 = 625 After Accelerating: energy = 50*50 = 2500 To figure out the round trip efficiency, we compare the kinetic energy that was removed during the braking process to the kinetic energy added during the acceleration. Kinetic energy of the car is reduced by 3,600 (4225 - 625) and then raised by 1875 (2500 - 625). In this case, I got 52% (which is 1875/3600) round trip efficiency on the deceleration/acceleration. These are just the numbers that I remember off the top of my head and also are not in any kind of a controlled environment. Also note that how heavy you press on the accelerator pedal has an impact on the round trip efficiency. As @LastGas mentions, it would be more efficient to simply coast and not use the regenerative braking. If it was in the Atom, I could take the corner at 55 mph and not need to brake/accelerate.

Once the Regen electrons are put back in the battery they are as good as any other electrons, so where does this 'double penalty' come from? Any heating of the brakes is completely lost energy, of course. And coasting above ~45mph renders that energy 'lost' as well. --

Kinetic energy of the car is reduced by 3,600 (4225 - 625) and then raised by 1875 (2500 - 625). In this case, I got 52% (which is 1875/3600) round trip efficiency on the deceleration/acceleration. But shouldn't you deduct the energy that would have been required to go the same distance w/o deceleration/acceleration. Maybe a bit difficult to test if it's a sharp corner.