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Let's continue with some ideas that may be helpful in putting together a very simple rainwater system using rainbarrels.

Multiple Barrels for Additional Capacity

Keep in mind that most homes typically have at least 1 downspout (or more) on each side depending on the home design. A single 50-60 gallon (190-225 liter) is effective for light irrigation but often is insufficient for larger gardens or homes with extensive plant life. The addition of more barrels, however, is a relatively straight forward task. One approach that is often taken is to locate a rain barrel under each downspout. This may also be desirable if you have more than one garden, or plants that require watering on opposite sides of the house. If you don't have multiple downspouts you can join multiple barrels together such that they fill simultaneously (connected by hose at the bottom) or so that one overflows into the next and so on.

Make sure your rainbarrel is also positioned appropriately so that you can fill your watering can from the spigot or attach a short hose to the spigot which will allow for filling of watering cans if you are unable to get the can underneath the spigot.

One thing to keep in mind, however, is that by the time you hook up 3, 4, 5 or more rainbarrels, and have to get the connector hoses etc. to make this feasible it might not be any more cost effective that actually purchasing a larger single system.  In addition, multiple rainbarrels typically use up a lot more space than a single tank system so keep this in mind.  You might think that your 5 rainbarrels is the cheapest way to put a system together but it might not be, and it might be costing you valuable time to setup and space around your home.  Check out the solutions here for an example of a great single tank, space saving design, that gives you a lot of storage capacity.

Proper Drainage Is Important!

A properly designed home has taken into consideration the importance of redirecting water away from the foundation.  When you install a rain barrel it is important not to overlook the importance of maintaining proper drainage.  First, ensure that you actually have an overflow drain on your rain barrel. If you don't, rainwater will just flow over the top of the barrel and run down right beside your foundation.  Make sure that the overflow on your barrel redirects the water in the same way your downspout did before you installed the barrel.  Your downspouts direct water away from the foundation for a reason, don't undo this by putting in a rainbarrel without a proper overflow.  It is important to note that many typical rainbarrels have a very small 5/8″ hose for an overflow. If you have ever looked at the volume of water that comes down your downspout in comparison to the volume of water than a 5/8″ hose can handle it becomes clear that in heavy rainfalls that little hose is insufficient.  In this case, even though you have an overflow it may not be appropriately sized to ensure that heavy rainfall doesn't cause the barrel to overflow from the top and just run down around your foundation. 

What About My Grass? 

So what about your grass?  Well, watering grass typically uses very large amounts of water, most of which, according to experts in this field, is wasted as people water too frequently, at the wrong time, or use too much water each time.  If you are looking to do irrigation of your grass a simple rainbarrel system is unlikely to be sufficient for what you need unless you have a fairly large number of rainbarrels connected together.  Just to give you an idea, many lawn care people recommend 1″ of water per week (note that this really does vary based on a number of factors but it seems for some reason to be a common number).  To put 1″ of water on 100 sqft of grass takes approximately 62G or 235L of water (this assumes 100% efficiency of the watering system).  For a 20ft x 30ft yard you would need 374G or 1415L.  That would be about 7 or 8 typical rainbarrels to store sufficient capacity for 1 weeks watering.  What if you don't have rain for 2 weeks? It might be appropriate to consider looking for a system with much more capacity and flexibility like the one here .  If you are over-seeding or putting in new grass, investigate native grass species that are more appropriate for your local climate.

Summary 

With a proper rain barrel system you may find, depending on your situation, that you can meet all of your basic irrigation needs with rainwater.  Remember, watering by hand and controlling where you water can be much more efficient than setting up your sprinkler to water your garden, and simultaneously your porch, sidewalk, driveway, car, and neighbour's children.  Remember that it is important to protect your water from contaminants and those pesky mosquitoes and to ensure your system has proper drainage.  Finally, think about your total system needs and potential cost.  Often the cheapest solution doesn't end up being the most appropriate and cost effective in the long run.