What Is a Watt Hour and Why Do I Need to Know?
If you are considering purchasing or building your own electric bike then you need to educate yourself a bit on different technical terms so you can make an informed decision when purchasing batteries for your bike.
One term that may come up is watt hours or Wh for short. A Watt Hours is an important measurement when determining how much battery capacity you require to travel a specified distance. It represents the total energy available in a given battery pack. It is calculated by multiplying the amp hour or Ah of a battery pack by the total voltage of the pack. A 36-volt 20Ah pack and a 72-volt 10Ah pack both contain 720 watt-hrs; therefore they should both be able to get you the same distance provided you ride at the same speed. With the 72v pack you can accelerate faster and have a much higher top speed than the 36v pack. If you take advantage of this (it's really hard not to use all that extra power) and ride at a higher speed you will encounter more wind resistance using up more power. This will result in your distance traveled prior to the batteries running out being less than that of the 36v pack traveling at a lower speed but you will reach your destination faster. With a Crystalyte 5304 hub motor in a 24-inch rim and a motor controller set to 90 amps you will average around 35 to 45 watt-hrs per km (58-75 watt-hrs per mile). This is at a cruising speed of around 65KPH (40MPH) and no pedaling. How fast you accelerate and your top speed will influence your power consumption. It will also vary depending on how flat or hilly the terrain is and how much weight you are carrying. You can use less than 25 watt-hrs per KM if you ride really slowly (using the same setup mentioned above) on flat terrain or use over 65 watt-hrs per KM riding full throttle up hills.
I hope this gives you some useful information on what a watt hour is and why you need to know. It makes it much easier to determine what size of battery pack you need to travel a specified distance.