Mega Evolutions work differently in Pokémon Go than in the video game series. Trainers can temporarily Mega Evolve a Pokémon to gain a power boost in battle and sometimes have its type change.
Mega Evolutions require Mega Energy in Pokémon Go. Each Pokémon has its specific type of Mega Energy, similar to Candy. The first time you Mega Evolve a Pokémon, it’ll ask for a high number of Mega Energy. But after that, the cost will be reduced.
To get Mega Energy, players will have to battle in Mega Raids. The amount of Mega Energy you receive corresponds to how quickly you finish the battle. The quicker you defeat it, the more Mega Energy you’ll receive. The type of Mega Energy you’ll get will be of the Pokémon you defeated.
Special Raid Battles featuring Mega Evolved Pokémon will randomly appear, with some events hosting some stronger Raid Battles. Mega Raids will be different from normal Raids, though—they’ll have their own difficulty levels.
It’s possible that Mega Energy might come as a reward in future Special Research tasks as more Pokémon become available to Mega Evolve, similar to how trainers can receive some Pokémon Candy after completing a task.
A Mega Evolution only lasts for four hours, though some events can have it boosted to last longer. During this time, trainers should battle in gyms and Raids. But Mega Evolved Pokémon can’t be placed in gyms or used in the Go Battle League for now.