Eva-Dry renewable dehumidifiers get damaging moisture out of the air of closets, cabinets, safes, darkrooms, boats, RVs - almost anywhere. These renewable units are filled with silica gel, a substance with a lot of absorption power. These odorless silica beads absorb many times their weight in moisture, locking it in so water can't leak or spill out. When the Eva-Dry is "full," users can renew the silica inside by simply plugging the unit into any power outlet, where it produces just enough heat to gently release the water vapor over the course of a few hours. When the indicator is blue again, it's ready to go back to work. The process can be repeated for up to 10 years! The EVD333 is recommended for rooms up to 333 cu ft, and should be plugged in every 20-30 days and the EVD500 is recommended for rooms up to 500 cu ft and should be plugged in every 30-60 days.