An outlet is found in both Chibi-Robo! and Chibi-Robo!: Park Patrol. It allows Chibi-Robos to recharge. There is one in the Chibi-House in Town, one on each Windmill, and several throughout the Sandersons' House.
Outlets are necassary for chibi robo to recharge, you will know when you should recharge once chibi robo starts flashing red.