Amazon pegs the Kindle Fire HD as offering 11 hours of use via its 4400 mAh internal battery – a step up from the 8.5 hours listed for the standard Kindle Fire. In our use, we found it landing more in the still-impressive range of 9-10 hours; and with the brightness cranked to maximum, it came in closer to about eight hours.
That's a very respectable sum for a tablet of this size, and 10 hours still puts it ahead of the Nexus 7, though it's roughly on par with the much pricier new iPad. You'll get a lot of on-the-go media viewing from the Kindle Fire HD.
Media storage may be another issue, based on what you're storing. As noted earlier, the device ships with 16GB of internal storage in the $199 model, while another $50 doubles that to 32GB. Both sums are double what is offered in the Nexus 7 – a definite perk, whatever your choice.
External storage is not an option here, though Amazon assumes you'll be using the device with a steady Wi-Fi connection as to make the most of the benefits of the cloud. Otherwise, be prepared to keep a close eye on the storage usage tally as you pick and choose your media and apps.