Just under four hours is a pretty good result. A nine-cell battery is available should you need more.
In this lower intensity test the Dell passes the four-hour mark. Thus, if you just need to read or write a document, you can do so for a long time between charges.
Just short of three hours at maximum brightness means even the longest films should be no problem. Reduce the brightness and you could be looking at three and half hours of playback time.