This has to be the most ecOnomical way I know to water my plants.
Plants LOVE the rain. They always seem to thrive more when I water them using this method. The fresh air also helps keep them healthy. BUT...you do need to keep in mind their sun tolerance. Just in case you leave them out when it stops raining and the sun comes out. Shade loving plants would not be happy sitting in direct sunlight!
So, my tip for you is:
Water your plants EcOnomically, set them out in the rain!