You'll need a valid time, so if you kill the app and come back offline, it'll need a valid time to be able to give you the energy generated offline, otherwise it'll generate energy while your in the game only.

For example, if you got 10 energy, you leave the game and kill the app (i.g. clean the background apps at one time, instead of deleting it), and you wait a week and come back offline, then the game will start generating the energy from 10.

However, once the game gets an internet connection, it'll know a week has passed, and give you full energy.

Thank you for your support! 😊