According to the Lower Colorado River Authority's informative website on how to save water in your home and landscape, "rainwater harvesting can be as simple as placing a barrel under a roof’s downspout for collecting water for your plants. Or it can be an elaborate arrangement of cisterns, pumps and filters to provide an entire household with water."
At one time, we had a rainwater collection system that held 17,000 gallons of water. We had two tanks like this that sat side by side and provided 100% of the water we used in our home and garden.
When we moved to town to downsize, we also had to downsize our system to three barrels that each hold 55-gallons.
During the summer, that's just a drop in the bucket (so to speak) compared to the amount of water we really need to keep our gardens alive. We don't like using town water since it's not really good for the plants (too many chemicals/chlorine) and is expensive, and getting more so as time goes by. Right now, the barrels are already dry and it's not even April. We've had very little rain here and the plants are thirsty!
We do plan to add more barrels, or hopefully, a larger tank which will also serve as a noise barrier between us and a neighbor on one side. The key is to have the storage in place so that when it does rain (please!), we'll capture every drop that hits the roof of the house and garage.
In the meantime, I literally place galvanized buckets in an around the garden, on the patio, etc. so that they also catch water.
An arty version of my supplemental rainwater collection system--inexpensive, decorative and portable!
I use them quickly so the mosquitoes don't breed in them, and you'd be surprised at how much you can catch that way. I also re-use the water when I change the cat's drinking water, catch water in the kitchen in a plastic tub that sits in my sink when I rinse my hands, and save cooking water (like from pasta, for example). It all adds up! The contents of our pasta pot, for example, uses one whole gallon of water. We don't add oils or anything while we cook, and after it cools, the plants seem to appreciate the drink of water with the wheat flour residue.
The point is...start small if you must, it all adds up! You can always add more buckets, or barrels, or tanks later. And by the way, there's a law in Texas that says that nobody can make rules that prohibit rainwater systems, including HOAs. And they can't prohibit things that go into a system, either, such as metal roofs, gutters, tanks, etc.
Please check out the LCRA website for more tips on rainwater harvesting, saving water in your home and water conservation.