In order to calculate the right capacity for a battery you need to do more than just multiply the energy requirement by the time. The number of Wh or Ah you have calculated is only what the application needs in order to run for a specified time. Important to understand is that the battery has some efficiency losses in charge and discharge. The battery needs to be overcompensated for this efficiency loss if you want to reach the required autonomy.

There are 2 major factors that impact the so-called efficiency of the battery:
- Discharge rate/Charge rate
- Temperature

1) Discharge rate and charge rate.
The faster you charge and or discharge a battery, the more internal resistance the battery will have. The internal resistance converts part of the electrical energy into heat. The faster you would like to charge or discharge the lower the efficiency and as a result the more heat generation you will have. This is a genuine problem for batteries and makes it also very difficult to compare the performance of batteries.
While reading the above you can come to the conclusion that if I discharge a battery in 1 hour, I have a different number of Wh in discharged capacity than when I discharge the battery in 5 hours. That's why within the battery industry we talk about rated capacity.

As the rated capacity is based on a discharge of 5 hours, it means that any discharge faster than 5 hours causes loss of efficiency, and as a result you will have less Wh in discharge. The effect of this is shown in the graphs below. The values mentioned are expressed as a ration of the rated capacity (e.g. a discharge of 5 hours means that the discharge current is 1/5 of the capacity value, this is described as 0.2C (1/5C)).