Our stacked rain barrels hold a total of 108 gallons of water when they are full, and at the start of this dry spell we've been having in Franklin they were full. We have them setup so they also collect water that our air conditioning unit produces every day, giving us a steady inflow of several gallons. At this point we're down to the last twenty or so gallons in the tanks, which will hopefully fill up again if it rains tomorrow.
The water barrels have been the only source of water for my borrowed earth boxes which combine to consume around 8-9 gallons of water a day. Those nearly seven foot tall tomato plants are thirsty beasts in this heat. We probably use another couple gallons watering the plants at the water station and Michelle's seedlings/transplants. With the additional input from the air-conditioner we've easily saved two-hundred gallons of water use that would have otherwise come out of the tap.
If everyone in Franklin took advantage of similar rain barrels to water their gardens with during this drought we could have combined to save a little over 7,500,000 gallons of water, which surely would have helped our over-burdened aquifer!
Many rain barrels I've seen only have a spigot a half a foot or so up from the base of the barrel, effectively cutting off many dozens of gallons capacity. While our rain barrels from Arid Solutions weren't cheap, they are very well designed, with multiple output spigots at different levels and overflow spigots at the top. The main source of water comes out of the base of the barrels through the green hose.
The fact that the spigot comes out the base of the barrels means we have access to all the water in them, and the other important aspect of our water station is the hose drops several feet to where we access it. This differential in height gives us great water pressure for a rapid flow. Filling a gallon jug takes about twenty seconds or so through this filling method, while using the brass tap mounted higher on the barrel takes much longer. If you do get a rain barrel consider placing it where you have a natural incline to take advantage of, or put it up on cinder blocks to give it a height boost.
The first year we had the rain barrels we didn't use them as much as we do now. Adding the hose that comes out of the bottom with a valve on the end of it made all the difference. It lets us take the hose where we want it and turn it on/off where we are. The hose and the valve cost a little extra, but combined with the height differential they make a world of difference in accessibility and flow rate over the generic spigot out of the side of the barrel. Happy conserving, and please everyone do a rain dance! ;)