I'll try Janie but no, I'm not experienced with them, it's something that built up over a few years, adding as new ideas came and a bit of money to spend on the garden was available. It started as Pip was finding watering more difficult.

The area that is watered by the rain water is a bit of land we rent in a field (there is a pipe to a septic tank that tractors break so this bit is unusable to them as farm land). As a guess, the main veg plot is about 20yds X 6yds. There are also 2 green houses, 1 8x4 ft and 1 (leanto type) 8x2ft. There are hanging baskets, beds (including 3 grape vines) around an area with 4 pigsties and there is another smaller veg plot.

The barrels are recycled plastic ones that once held mango chutney! Say about 45 imperial gallons each. They are in 2 locations. The first set of 8 collects from the roof surfaces of our bungalow. The remaining 4 are located in the field and collect from a shed we call "the wendy house". The sets of barrels are about 30 yards from each other and are connected together by a 1" plastic pipe which runs behind the pigsties. (I could add extra barrels in the middle of this pipe run).

The we use spray and drip watering and as well as plain watering, can apply soluble fertilizer to the areas. the water is pumped.

There is an 80W solar panel on the wendy house and this (as well as a pond pump and at one time some LED lights) powers the pump. I use a marine freshwater pump like this one

The water is turned on by tap timers. I don't use the automatic time of day features but I do use them as they water for a set time. It's just a case of setting them to manual and pressing a button and the timer will water the areas for the programmed durations.

As for how long the water lasts... Apart from fertilizer, we only really water when things look as if they need it and this year, a lot of the time it looked in need - particularly the young seedlings that were wilting a lot...

We don't water for long (and I'm not sure how long the battery in the solar system would do on a dull day...) but water each area on average about 5 minutes or 10 if I run it twice, etc. and I think the plants do appreciate a proper rain soaking when the rain does fall.

Last year, the rainwater lasted. This year, I was out by June but subsequent rain has filled the butts again - wasn't far of lasting the dry spell out.

Sorry it's so vague but I hope it gives a bit of a picture for you.