I have a 30 gallon rain barrel in the basement. It was originally for a backup rain water supply for my Bonsai and house plants, but since I got a RO filtration system I don't seem to need it very often. Potted plants don't like hard tap water, and that's what we have. I keep an aquarium filter in it to keep it fresh. I run it for a few days every couple months, and top it it up with RO water.

I have another rain barrel outside in the summer, but that's in frequent use so I wouldn't count on it for survival.