How much energy loss would be if I used generator and motor to propel a bicycle instead of pedaling directly?

I'm thinking of making a lightweight scooter that can be propelled by electric motor, but can be pedaled if the battery is gone. Instead of making the pedaling component main function, i want to pedal to store/generate electricity. Does this physically/electronically make sense, or would there be too much energy loss? What if I can find very efficient motor and generator?