I think there should be more ways to gain energy, or even upgrade how much energy you can hold.
1st way to gain: Minigames
There should be minigames that you can play once, twice, or thrice a day (depending on how long you've been registered) that you can play to earn energy. [Awards: 1st- 20 energy / 2nd- 15 energy / 3rd- 10 energy / 4th- 5 energy
1st way to upgrade energy(ignore 1st gain idea if this is used): Growing mist tank
Depending on how long you have been registered, you'll be able store more energy, but wait longer to fill the extra room
e.g.
24 hours 100 energy
48 hours 200 energy
And so on...

2nd way to gain/upgrade energy
Using the levels on items(not counting default items), everytime you reach level 10 with an item you can choose to gain a some bonus energy, or gain extra room for energy. (Suggested use bonus energy, but both are fine)

(These ideas are for MIST energy)