The DIW-20 provides approximately 300 gallons of mineral-free, de-ionized water based on your water input quality. It consists of two 20 inch de-ionization housings that feature replaceable (or refillable) cartridges that simply slide into the watertight tubes. The entire unit is mounted on a powder coated, rust resistant bracket with pre-drilled holes that allows for easy mounting to provide years of worry-free (and *spot-free*!) use. The advantage of using this system is that you can rinse your vehicle and walk away without having to worry about water spotting - no drying towels required!