Board Thread:Naruto Discussions/@comment-12509038-20140806073433/@comment-9782468-20140807035955

Itachi had horrible chakra levels too, but he had the sharingan activated almost all the time, only deactivating it when using the mangekyo. His chakra level was lower than Kakashi's. It's listed at 2.5, while Kakashi has 3 (cited from Itachi's page). While it did drain a lot because he didn't deactivate it, the sharingan drained Kakashi mostly because he couldn't actually awaken it like an Uchiha. His body couldn't handle the eye like a normal Uchiha could.