From Bulbapedia, the community-driven Pokémon encyclopedia.
The Thunder Meter (Japanese: 電撃メーター Electric Shock Meter) is a game mechanic in Pokémon Pinball.
In Pokémon Pinball, the Thunder Meter displays how much electricity is stored for the Pikachu Kickback (a Pikachu that returns the ball at the bottom left and bottom right corners of the field). The Thunder Meter starts empty, but is charged when the ball spins the Spinner by passing through the Right Loop. The Pikachu Kickback can only be used when the Thunder Meter is completely charged. The Thunder Meter is emptied again if the player uses the Pikachu Kickback or loses a ball.
When the Super Pikachu Kickback (an improved version of the Pikachu Kickback that protects both corners at once) is active, the Thunder Meter remains indefinitely charged. The player is able to use the Super Pikachu Kickback to repeatedly retrieve the ball without emptying the Thunder Meter. However, if the player loses a ball, the Super Pikachu Kickback reverts to the regular Pikachu Kickback and the Thunder Meter is emptied.
Pokémon Pinball Ruby & Sapphire
In Pokémon Pinball Ruby & Sapphire, there is no Thunder Meter anywhere in the playing fields. The Thunder Meter was replaced by a thunder icon displayed at the bottom right corner when Pikachu is charged.