As for filling the 20# tanks, there is a difference between having them filled at a propane suppler/dealer vs. a gas station, big box store, convenience store, etc. The dealer will fill the 20# tank to 80%, or 16# (assuming it is empty). The exchange places are usually only filled to 75%, or 15# (that's part of their profit margin). That extra one pound might make a difference if you are running low and need to finish something right away. Also, if you continue to use the big box exchange method, the difference (loss) becomes cumulative each time you swap out if you aren't taking in a completely empty cylinder.