While the iPad surely looks like a great device for reading books or for surfing the web from the couch, very few people outside Apple (except Stephen Colbert) may have had a chance to use the Apple tablet first-hand.
Therefore, whatever little we know of iPad’s battery life is based on the technical specs that say — “up to 10 hours of surfing the web on Wi-Fi, watching video, or listening to music” — followed by this lengthy disclaimer:
Testing consisted of full battery discharge while performing each of the following tasks: video playback, audio playback, and Internet browsing using Wi-Fi. Video content was a repeated 2-hour 23-minute movie purchased from the iTunes Store. Audio content was a playlist of 358 unique songs, consisting of a combination of songs imported from CDs using iTunes and songs purchased from the iTunes Store.
Internet over Wi-Fi tests were conducted using a closed network and dedicated web and mail servers, browsing snapshot versions of 20 popular web pages, and receiving mail once an hour. All settings were default except: Wi-Fi was associated with a network; the Wi-Fi feature Ask to Join Networks and Auto-Brightness were turned off. Battery life depends on device settings, usage, and many other factors. Battery tests are conducted using specific iPad units; actual results may vary.
Thankfully, you don’t have to read through this boring disclaimer as Jobs recently offered some direct hints on iPad’s battery life to WSJ’s Walt Mossberg.
Steve says that iPad will offer ~140 hours of continuous music playback provided the screen display is turned “off” while the battery will last for ~10 hours if you use it for reading books. Steve says that the iPad chip [A4] doesn’t use much power but the screen does. I guess the numbers would be similar for web surfing as well.