The final part of harvesting rain water is storage. It comes down to a simple question: how much water do you think you’ll need and for what. For example will you use the water just on your lawn, flowers or vegetables? Will you also use it for washing windows or the car, for flushing the loo or in the kitchen?
You can buy some pretty fancy water butts (I have a low threshold for what I consider fancy) that are designed to blend into the background. There are taller thinner tanks that are excellent for saving space if you don’t have much space to spare but you can be looking at €200 at least for one of these. To keep costs down, if you just want an extra couple of hundred litres stored, a smaller amount than you might think, you can simply add another barrel to the filter barrel. If you think you might have use for more water or are intending to use the captured rain for drinking, cooking and showers then you might want to go larger. Of course you’ll need a pump too in that case.
Looking around the web I was able to find several 1000 litre water tanks for between fifty and a hundred Euro. They are generally not used to store drinking water as they are not lightproof and light promotes microbe growth, but they can be clad or kept in a shed. A thousand litre tank specifically for potable water can cost upwards of five hundred Euro. An important detail about storing larger amounts of drinking water is that without some ongoing sterilisation, there is a chance of bacterial contamination. This isn’t necessarily a danger but a strong UV filter light can easily be fitted and it has the benefit of keeping unpleasant bleachy chemicals out of your morning tea. The amount of equipment increases with domestic use but the kit is usually a one-off purchase and other than the filter media (sand, gravel, charcoal or even just sand) and a UV bulb you’ll likely never have to replace any of it.
If the water is not being used for drinking and cooking, this last step is unnecessary but you still need to think about keeping the tank a little higher than ground level so you’ll have enough pressure to water the veg.
Finally, a word about maintenance. It’s recommended to completely change the filter materials in your barrel every couple of years, regardless of the type. If you clean the mesh screens at the mouth of the downpipe every few months it goes a long way to keeping the water clean but remember the charcoal and sand will be filtering out a lot of very small particles. If you have no filter, it’s still recommended to disconnect and scrub the barrel every two or three years.