Bumping this topic up, since it's the middle of the summer and I just ran some numbers on our rain-barrel water use...
We installed two RBs at our new house this spring. One of them has a capacity of 65 gallons and the other, 55 gallon.
We use the water from the barrels regularly to water our garden. It's definitely slower than watering from the tap and hose,
but I view this as a feature rather than a bug most of the time. I like to "visit" with the plants, see how they're doing--one
of the things I like about gardening is the excuse to slow down. We do occasionally water from the tap when it's a really hot
spell and we need to make sure everything is well-watered quickly before we leave for the day, something like that.
We've seen the larger barrel go from full capacity to completely empty two times this year so far. Since it takes a while to
go from full to empty, and rainwater still accumulates when the water level is somewhere in the middle, I guesstimate that
going we actually draw about 100 gallons from the 65 gallon tank as it fluctuates along the way from totally full to totally empty.
So, I guesstimate we've used 200 gallons from the bigger tank.
The smaller tank is used about half as much as the bigger one, so I'll estimate that we've used 100 gallons from it so far. That's a
total of 300 gallons, more or less.
Looking at my last water bill, reflecting May and June of this year, our household (3 adults and two kids) averaged 110 gallons/day.
So, the rain barrel water for our garden this spring and early summer has saved us an equivalent of 2-3 days of water use.
Depending on how you break it down, this comes out to $5-7 on our water bill. Assuming that we'll use about as much water for the rest of
the growing season (and in Seattle, we can be fairly sure the water in the barrels will be replenished even through August), that saves us
like $12 in water a year.
I paid $130 for the two barrels, so this calculation argues they'll pay for themselves in 11 years. That's a long timeframe, but not infeasible.
We put in two essentially identical rain barrels at our old house probably 8 years ago. They're still in heavy use, working fine and and looking great.
Anecdata, perhaps, but data nonetheless! I still love our rain barrels even if they are a long-term investment. They are an important part of our yard's drainage-management system, teach our kids a few things about natural resources, and give me an excuse to slow down. Priceless!