Denim brings nothing for the Lumia 920. Can you expect Denim? I'm not sure.
Owners of newer models should though.
Windows Phone 8.1 Update 1 and Lumia Denim is veyr minor and won't magically fix battery woes. (sign up for the Preview for Developers program if you want it because Lumia Denim doesn't really do anything more than Cyan as far as you're concerned)
1. Do the standard battery saving spiel. Check battery saver, account sync frequency, radios, are you gaming often?
etc
2. Battery may need replacement. They wear out and need replacement - no software update will compensate for that.
3. you can wipe the phone and start clean if you'd like, it often fixes some issues.