When it's worth to suck citizen's blood in Vampyr?

In Vampyr you play the role of Jonathan, a man who has been transformed into a vampire. Thus, you are able to bite your victims in the neck and suck blood. This strengthens your character, causes a serious increase in experience and... kills the misfortunes that have fallen into your hands. In this chapter you will find out when it's best to suck in the blood of the people who live there.