The simplest and most popular system is a small bin or trash can placed under the downspout gutter with a tap near the bottom for attaching a garden hose. It’s great for starting out, casual users, or homes with lots of downspouts and a garden to water around the house. Water can be easily distributed and little construction is required to install the system. However, storage capacity can be limited with the single barrel approach and our house is built on top of the other two, limiting the number of downspouts and external storage space.
Another approach is to use a large IBC or water tank on a single downspout. The storage capacity is greatly increased and the user gets more water pressure (at least when the tank is full enough). But again, this wasn’t great for our use as we only had one main downspout near our garden space, and only a narrow space between the stairs and the back of the house to house rainwater storage.
These considerations lead me to a third system, daisy-chaining multiple barrels. One barrel serves as the main input, but several other barrels are connected by pipes in such a way as to allow the barrels to fill at the same time and rather evenly. This would allow us to use a narrower barrel that would fit in our space but still increase capacity. In theory, an unlimited number of barrels could be connected and the rainwater storage capacity increased as finance and space allowed.