@Fire makes a very good point how fast water butts empty over a dry period. I have a single 300L butt; I limit its use to plants that don't like our hard tap water, mostly ferns. If I used it for all my watering needs (mostly pots, seedlings, and the newly planted, I do actually try to have a fairly drought-resistant garden) it would empty within two days during a hot spell.
It isn't always necessary to use pumps to move water around, syphons will do the work if you have the right levels in your garden. Bob Flowerdew is a great advocate of this technique. Then you can distribute the contents of the water butt connected to the downpipe to storage around the garden. Storage could be extra water butts, troughs or even rubber trugs. That will effectively multiply the storage capacity and save you carrying water around to where it is needed.
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