The key to any electric bicycle is the battery and how much energy it can hold. The battery is the gas tank. The battery holds energy. Power accelerates the car, but the size of the gas tank (energy capacity of the battery) determines the distance.
A kWh is a unit of energy equivalent to one kilowatt (1,000 watts) of power expended for one hour of time, or 1,000 watt hours (Wh). This is the way you buy electricity in your house. Your house electricity bill charges you for the amount of kWh of energy you use in the month. The best electric bicycle will have a larger battery, measured in kWh.