Advanced options
Roof area is the footprint the downpipe drains: half a typical semi's roof is 25-40 m2. UK rainfall runs ~600mm (East Anglia) to 1,500mm+ (west/uplands). The practical cap is butt capacity in dry spells, which is why serious gardeners chain butts. Plants prefer rainwater anyway, and it's hosepipe-ban-proof.
What The Water Butt Savings Calculator Works Out
- Collectable per year: roof area x annual rainfall x 80% collection efficiency. 40 m2 under 800mm of rain sheds 32 m3; a downpipe diverter realistically banks about 25,000 litres of it. UK rainfall runs from about 600mm in East Anglia to 1,500mm plus in the west and uplands.
- Butt refills: the collectable water divided by the butt's capacity. A 210 litre butt refills over a hundred times a year in theory, but stands empty in exactly the droughts it exists for, which is the argument for linking butts.
- Metered water value: the collectable volume at GBP 3.70 per m3, a typical combined metered water and sewerage rate. The real prize is hosepipe-ban immunity and soft, chlorine-free water that ericaceous pots such as blueberries and camellias prefer.
- Collection efficiency (advanced): edit the 80%. Steep, smooth roofs with a good diverter can reach 85% to 90%; mossy roofs, leaky gutters and first-flush losses pull it down to 60% to 70%.
- Water price (advanced): your own metered rate from the bill. Unmetered households save nothing in cash, only in resilience.
- Number of butts and total storage (advanced): linked butts multiply the storage. The card shows how many weeks of your watering a full set covers with no rain, the practical measure of whether the system survives a July dry spell.
- Rain needed to refill (advanced): the millimetres of rainfall that fill the empty butts from your roof. A 210 litre butt on 40 m2 refills with about 6.5mm, a single wet day, which is why a bigger roof area beats a bigger butt.
- Realistic annual saving (advanced): what you would actually use, capped by your watering demand and by the storage available each week, at your water price. For most gardens this is GBP 10 to GBP 25 a year, well below the headline value of all the rain that falls.
- Payback (advanced): the set-up cost (butt, stand and diverter, about GBP 40 to GBP 60 each) divided by the realistic saving. Many water companies subsidise butts in dry regions, which shortens it. The table compares butt sizes and one extra butt for your roof and watering pattern.