Google promise of Android L increases battery life seems true, because when Google Nexus 5 was loaded with preview of latest Android L, it's battery life significantly increases than in current Android 4.4 KitKat. Google Nexus 5 with Android KitKat was charged up to max, On standard browsing at 200 nits of brightness it survives up-to 6 hours and 45 minutes before battery died completely.
After that when device was loaded with latest Android L Developer preview, it gains battery life significantly, and on same usage of web browsing device survives nearly 8 hours. This test also proves the Google advertised fact that latest Android L increases battery life up to 15 to 20 percent.
Android L comes with Google new Project dubbed as Volta, that aims to optimize the battery consumption, and gladly it works as advertised by Google. Many things under this project are on developer side of things, it also get visible perks on user end as well. One of the visible bonus on end user side is Battery saving mode, which automatically starts when battery reaches on 15%, while it can be enable by users from setting menu.
Although change in battery life is pretty significant but is not breathtaking, but still we can expect more from final version of Android L. would You like to upgrade your devices from Android KitKat to Android L or not ?