For most electric all-terrain bike manufacturers, the battery capacity is indicated in AH (Amp Hours). This indicates how much capacity the battery has. Technically, it measures how much current your battery delivers in one hour of usable voltage. As an example, if you have a 14AH 36 Volt battery, you'd have a 504 watt hours battery. In simplest terms, this means your 500 watt output motor could run at its highest level of assist for one hour. Or, you could run it for two hours using only the mid level of assist, and so on. 

Here are things that can have a positive or negative impact on battery life:

• The speed you are riding
• Your level of assist (low level to full throttle)
• The typography of the terrain 
• Weight of the cargo being carried
• Weight of the rider
• The intensity of the tailwind, headwind, or crosswind
• Number of starts
• The motor size & efficiency  
• Hub drive or Mid-drive
• Ambient temperature
• Tire pressure/size
• Size, maintenance, and age of the battery
• There can be several other factors, but these are the most significant

As you decide the output of the motor you’d like for your electric bike, the consideration of the battery is almost as important. For example, if you plan to purchase an electric all-terrain bike with a 750 watt motor. You are best served to get a battery that is at least 48V and 15.6AH. This will provide you with one hour of max output from your motor. So, you will have at least 2 hours (most likely more because of the various factors listed above) of available battery by using the pedal-assist up the mountain but no assist down the mountain for example. Simply multiply the voltage by the amp hour rating of the battery to acquire the max output. In the example here, its 48V x 15.6Ah = 748.8 Watt Hours. Let’s say you selected a 21Ah battery instead, then it’s 48V x 21Ah = 1008 Watt Hours. Which is just over 33% longer range.

There is also a school of thought that mid-drive motors generally get more range when motor wattage and battery size are the same. Mid-drives tend to work more in “harmony” with the gears because of their location compared to the hub drive where the gears of the drive-train work independently. Higher end mid-drive motors also have advanced torque sensors that help with battery life.