A "water panel" is really the same humidifier part as a "humidifier water pad" or "humidifier filter" - used in a whole house humidifier. Traditionally, the water panel is a metal mesh pad dipped in a liquid clay slurry. The clay coating absorbs and holds water for distribution into the air. Other whole house pads are made of highly absorbent paper with aluminum reinforcement. Metal and paper pads work the same way - but paper pads should be replaced twice as often as metal pads.