In a modern car, using a moderate amount of electric power from the car could amount to zero increase of the fuel consumption.

How this is possible?

The car alternator is kept off most of the time. It is switched on only when the engine is used in "brake" mode, when the wheels transfer power to the engine and the fuel is cut off. Most of the kinetic energy dissipation happens in the engine, but the alternator can get as much as it can.

In short, in this very moment the mechanical energy is for free.

Of course, this works only if enough engine braking is done, e.g. in an intensive urban cycle and by an experienced driver. Otherwise, when the battery depletes past certain point, the alternator is engaged when the engine is providing mechanical power.

In a modern car, using a moderate amount of electric power from the car could amount to zero increase of the fuel consumption.

How this is possible?

The car alternator is kept off most of the time. It is switched on only when the engine is used in "brake" mode, when the wheels transfer power to the engine and the fuel is cut off. Most of the kinetic energy dissipation happens in the engine, but the alternator can make use of as much as it can.

In short, in this very moment the mechanical energy is for free.

Of course, this works only if enough engine braking is done, e.g. in an intensive urban cycle and by an experienced driver. Otherwise, when the battery depletes past certain point, the alternator is engaged when the engine is providing mechanical power.