It could be that simple. It depends on what you want to do. In planning a battery-operated project there's a few things you can look at:
- How long do you want it to run? (hours)
- How much power does the thing use on average? (milliamps)
- Multiply #1 and #2 to get a battery capacity in mAh.
- Find a battery that big.
- Decide that battery is too heavy - you can either reduce power consumption or run time.
- Is the battery supposed to be rechargeable? How will it be charged?