How to get more Spell Energy
Spell Energy functions similarly to Poke Balls in Pokémon Go. You will use one of them almost every time you have some kind of confrontation. In Wizards Unite, you use Spell Energy each time you fight another wizard or cast a spell on a Foundable to retrieve it.
The default amount of Spell Energy you can carry at any one time is 75 (more on this later), and it costs one Spell Energy for each spell you cast (though, it can take multiple attempts to capture a Foundable since they frequently resist your spell, especially when your character is at lower levels).
There are two primary ways to get more Spell Energy. The easiest is to find an Inn. Walk to one and enter. You'll be offered what the game refers to as food. The plates will shuffle themselves, then you cast the spell presented by the game. When successful, you are rewarded with Spell Energy. You'll collect anywhere from 1-10 Spell Energy per visit. (At least, those are the confines we've seen so far.) You then have to wait five minutes before you're allowed to interact with that same Inn again. You can, of course, go to another Inn inside that five-minute window. This is the same wait period you'll encounter after entering a Greenhouse.