The optimal build I found is an internal geared motor (they are smaller / lighter), and under 3kg. That, plus the battery pack I got weighs around 3.5 kg, and

The optimal build I found is an internal geared motor (they are smaller / lighter), and under 3kg. That, plus the battery pack I got weighs around 3.5 kg, and is 700 watt-hours, which is enough for around 40 - 60 miles of assist at 15 - 20 mph, assuming you are putting in enough peddling that would normally get your bike to about 10 - 12 mph.