From Bulbapedia, the community-driven Pokémon encyclopedia.
Watts are a type of currency used in the pedometers Pokémon Pikachu, Pokémon Pikachu 2 GS, and the Pokéwalker. Watts are represented by 'w'.
In Pokémon Pikachu, 1 w is earned for every 20 steps taken. Watts can be spent on gifts for Pikachu to increase its friendship, which unlocks additional games.
Pokémon Pikachu 2 GS
In Pokémon Pikachu 2 GS, watts can be spent on gifts for Pikachu, or sent to Gold, Silver, and Crystal or other Pokémon Pikachu 2 GS games.
In order to send them to a Generation II game, they must be sent through the Mystery Gift feature. The Pokémon Pikachu 2 GS game selects the amount to be sent, and the player of the Game Boy Color game receives one of the following gifts depending on the numbers of watts sent:
Watts can be spent on the Poké Radar and on the Dowsing Machine. It costs 10 w per use of the Poké Radar and 3 w per use of the Dowsing MCHN. When connecting to HeartGold or SoulSilver, all unspent watts will be transferred to that game. These watts are stored on the game card and unlock new Pokéwalker Routes when enough are collected.