We didn't get rain for a couple of weeks there in April. But when we woke up one Sunday morning, the ground was wet and it was obvious it had rained a little the night before. I walked past the rain barrel on the way to church but resisted checking it until I got home.
I opened the first barrel: full.
Opened the second barrel: full.
Whatever rain we had gotten that night had completely filled both barrels, about 60 gallons' worth.
Rainy Days and Math Days
The standard equation is that 1 inch of rain on a 1000 square foot roof will be about 600 gallons. Our roof is about 480 square feet, so we can collect about 288 gallons per inch of rain. That's a lot of water! Obviously, the limiting factor here won't be how much there is to catch. The problem is how much can we store and use?
So I looked at what we could realistically get out of our rain barrel. First, you toss the winter months out, because you can't use a rain barrel in winter where it freezes...the ice will kill it. We get about 28.78 inches from April to October, or about 4.11 inches per month. Assuming you collect all of that that's 1184 gallons per month or 8288 gallons total.
That's the capacity anyway. Realistically, you'll never collect that much because you'd have to have tanks to store it. What's actually going to happen is that you'll collect the first 60 gallons and then the rest is overflow. If you then have a dry period, you'll gradually use your 60 gallons until it rains again and, hopefully, replenishes your supply.
Let's use this April as a model. April 1st we had .36 inches, which is easily enough to fill our 60 gallons. We had plenty more rain all that week, but we wouldn't have had any more room to store it, so none of that really counts. Plus as long as it was raining, we wouldn't need to water the plants anyway. The week of the 6-12 was quite a bit dryer. Here we could have gotten some good use out of the rain barrel, and the .13 inches on the 12th would have topped us off somewhat. Then, 2 weeks of 0 precipitation--a dry spell. Here's where the rain barrel would have gotten some serious use. I'm not sure, at this point, if the 60 gallons would have lasted all this time. I think so, but I'll have to see as I use my barrel more in our garden. At any rate, the very last weekend of the month was a soaker, with about 2 inches of rain.
A Little About Rainbarrels Saving Money
The sobering thing about all this for me is that you really have to use a lot of rain water to save money, at least where we live. I looked at our water bill and calculated that water was costing us .36 cents a gallon. Note the decimal point--that's not thirty-six cents, but 1/3 of a cent! Municipal water is extremely cheap here. The 60 gallons in my rainbarrel now would cost me 22 cents from the spicket. Not a heck of a lot. Assuming about two refills a month from April to October inclusive, that's a little over $3.00 saved a year. At that rate, it would take me about 13 years to recoup my rain barrel investment!
I could add barrels to collect even more, but I'd have to factor in that expense. Plus, how many gallons can I realistically use in a season? It reaches a point eventually where I just can't use the water anymore. The long and short of it is, using rainbarrels for a small garden in this particular area is not a great way to save lots of money. It might be more feasible if municipal water is very expensive where you live and if you get enough rain to add up to considerable savings.