The power generated by this is probably several magnitudes smaller than what a phone uses. What's the use when your phone gets e.g. 0.1% more energy efficient?

The power generated by this is probably several magnitudes smaller than what a phone uses. What's the use when your phone gets e.g. 0.1% more energy efficient? Regenerative braking makes sense because it's a lot of energy and you can use the motor as a generator.