Vinicius B
The idea would be to have a second energy bar, from 1000 to 2000 for example.
This second bar could have a different regeneration rate (half the normal value) and would be useful for those who were unable to play for a few days to not "lose" the energy from the days they were unable to log in.
I don't think this would harm grinders, as there would still be a limit to how much energy you can store (2000).
Honkai Star Rail have a similar system: https://honkai-star-rail.fandom.com/wiki/Reserved_Trailblaze_Power#Replenishment