There was no option in iphone to show the remaining battery level in top status bar before iOS 4 came, but however it was possible after jailbreaking the iphone. But with the release of iOS 4 apple has implemented this feature in the firmware as a feature suggested by most of the apple iphone users.
So, now if you are running iOS 4 on your iphone or ipod touch you can display the remaining battery indication in terms of percentage on the top right of the top status bar on iphone.
Follow the procedure below to show battery percentage in top status bar
1. Go to Settings >> General
Now tap usage
Under usage tap the option to display battery level indicator in top status bar