Martin's toilet rain water harvesting spreadsheet

Find out how much water and money you flush down the toilet every year!
Put your household details in the white cells of the second column in the spreadsheet below. You can find typical values from the tables underneath the spreadsheet or the hyperlinks within the spreadsheet.

The default values in the spreadsheet are for a 2 bedroomed house in Anglesey, North Wales. Apologies, I havent found a way of keeping the calculated costs to 2 decimal places.

This sheet was built using Trimpath and Javascript

Put your household details here
Annual average rainfall (mm) 850
Roof area (m2) 30
Fraction of rainfall collected by your roof (0 -1) 0.8
Toilet flush volume (litres) 6
Toilet flushes a day 10
Price paid for water and sewerage (£) 2.53
. .
Results .
The most water on average that can be collected ('000 litres per year)
Tank storage needed ('000 litres)
.
Water flushed per day (litres)
Volume of water flushed ('000 litres per year)
. .
Cost of water flushed a year (£)
The most that could be saved a year using rain water harvesting from your roof (£)


Roof Area
Roof type Roof area range (m2)
Shed 2 to 20
2 bed house 20 to 35
3 bed house 30 to 45
5 bed house 45 to 70
Chapel 60 to 150


Roof collection coefficient
Material Steep (>45%) Shallow (45-20%) Flat (<20%)
Slate, tile, metal sheet 0.98 0.8 0.6
Shingles 0.7 0.6 -
Felt and Gravel - 0.4 0.2
Bare ground 0.5 0.2 0.05

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Toilets
Toilet flush volume (litres) Flush type
<6
Very low flush
6
Standard low flush
9
Pre 2000 standard flush
>9
old fashioned toilet cistern


Tanks
Tank Sizes Tank type Price Guide
50 (litres) Jerry can
£10 - £20
220 (litres) Water butt or oil drum
£20 - 30
1 (cu. meter) Industrial bulk storage container
£40 - 60
1 - 10  (cu. meter)
Commercial rigid tanks
£300 - £2000

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