The min energy is 90 (Natasha), and the max energy for most characters is 140 (Jingliu, Yanqing). And then Argenti has a unique energy mechanic and can store up to 180 (double burst).

Using basic attacks, E, Q can generate energy. It seems that unlike Genshin, in HSR characters can gain energy from getting hit by enemies. This introduces the following idea for Genshin: have a character generate energy from getting hit, taking damage, or draining hp. Similar to how Marechaussee gives crit from hp fluctuation, it should not be too hard to design a passive, kit, artifact that gives more energy from hp fluctuation. If an artifact is made for this, it can bring this idea to the masses. It could also turn Focalors into a battery (generating energy, besides being a crit rate buffer through Mare).

Note just because Focalors can drain team hp (or healers like Jean can replenish team hp), that does not necessarily make Mare the new bis for characters. Even for someone like Xiao who drains hp, Vermillion edges out Mare. Similarly, if an energy hp fluctuation artifact set releases, that might not necessarily make it the new bis for existing characters just because Furina, Jean, etc. can manipulate team hp.