I have three 50 gallon rain water collection barrels and have never fully used them for irrigating my garden, as much as I'd love to. Gravity pressure alone is not enough to use any sort of automated drip system or anything and we've been left having to use watering cans when using the collected rain water. The barrels do fill up quickly after one or two good rains.

I guess my point is to make sure you have a sufficient pump design in mind to use the collected rain water. Best of luck.

It's been a while since I made this post, but I've finished my rainwater harvesting / irrigation system, and the advice I received here was definitely helpful. I learned some things, too.

• More water storage was the right call. I had initially thought 800G, but I ultimately went with 1650G. Lately, my flow meter has been showing about 100G per day of water usage. I'm watering a large patch of grass seed, so I expect the usage will go down significantly once the grass germinates and I can stop watering so frequently, but I had no idea I would use so much. I've checked and double-checked the math a dozen times, but it's all correct. New grass uses a lot of water. Just to put it in perspective, my grass area uses 3GPM and gets watered 4x per day for 7 minutes each time (and the grass seed still starts to dry in our 90-degree heat). That's 84 gallons of water per day just for the grass. Kind of makes me wonder if grass is worth it, to be honest.
• I bought the biggest pump I could afford: 2HP with a ~70GPM flow rate. It was way too much; restricting the outflow caused the pump to work way too hard, so I had to build a re-circulation system to pass a large portion of the outflow back into the water tanks.
• Algae growth in the water tanks hasn't been the issue I thought it would be. My tanks are positioned in an area that's blocked by a building to the west and tall trees to the east, so they never get sunlight.
• Filtering is absolutely essential. Even with a 6ft section of pipe that catches the initial runoff, the first rainfall still filled the tanks with dirt from my roof. I have a 150 mesh spindown filter followed by a 20-micron canister filter, and I'm very thankful I decided to go with a 2-stage system instead of just the spindown filter.
• IBC totes are a huge pain in the ass. The outlet on my tanks is not a standard threading, and adapters simply do not exist for the particular type of fitting I have; I had to use a rubber coupling between the tank outlet and my PVC plumbing. The tanks expand and shift as they fill, so the plumbing has to have some built-in flexibility. I lost count of how many rubber couplings I had to use, but it was a lot. Bulkhead fittings will leak, so they have to be placed on the top of the tanks with internal plumbing from the fitting to the tank bottom. Using a large purpose-built water tank would have been the best choice, but it just wasn't an option financially.

All-in-all, though, it was a really fun project to work on. I went with a Rachio 3 sprinkler controller, Orbit valves, and ran buried pipe to all the areas that needed water. Everything just waters itself now, and I know exactly how much water each area is getting. Not only does nature provide the water for free, but I'm also not wasting potable municipal water either (at least not as much; I've still had to fill the tanks from the hose a few times). It was well worth the work and the expense; thank you again to everyone who posted here to help me out.

Depends on what your cost of water is. Setting up rain water reservoirs, plumbing it to your garden, and cleaning/maintenance adds up quick. If you’re only watering a small garden, might just want to stick with the hose.

I’ve got about 800gal capacity, and I use it all. My garden is large, and all the excess gets dumped into the pond.

Your garden will get bigger. If it’s legal (where I’m at in the PNW, it’s not exactly legal to do) I’d harvest as much as you can. You’ll find uses for it.

Where I live, apparently its illegal? Not rly sure why.