Comment by wildzzz

The dynamo wouldn't necessarily need to provide full power. Say your commute is 1 hour. The dynamo can provide 3W and the light draws 5W. Your morning commute would provide 3Wh to the battery. You don't use the light so all power charges the battery. During the evening, you now have at least 3Wh of battery alongside 3Wh of pedal power so you'd have 1Wh of battery left by the time you get home. A large enough battery could store hours of surplus light just based on your normal daily commute. If not battery power is available and it's night time, the light could be dimmed or you could charge the battery from USB. If the light is off, then load on your muscles from the dynamo would decrease as the battery charges to full.