teaching notes 5 - calculations - water used and savings made
Ref: T05

The figures given for the monetary savings on each of the reminder sheets are based on water metered at a cost of 0.086 pence per litre. This does not include other costs such as a standing charge and sewage costs associated with the supply and disposal of household waste water.

South West Water Ltd currently charge £0.86 (86 pence) per cubic metre

1 cubic metre = 1,000 litres

1 litre costs 86 pence/1,000 = 0.086 pence (£0.00086) per litre

The calculations on the following pages show how each of the reminder’s figures were arrived at. In general the amount of water used and savings made are calculated for an average family of four over the course of a year where appropriate. The cost of heating water is not included in these calculations, but would offer increased savings.

Note: Published water use figures vary from organisation to organisation. Also quantities will vary according to size of bowl, glass or capacity of washing machine, etc.

As an additional exercise students could be asked to measure how much water in litres is used in a washing-up bowl, bucket or watering can, etc.

Re-Use Water

A full bath uses around 150 litres of water X 4 (average family) = 600 litres per day X 153 (days in a five month period) = 91,800 litres of water which could be saved and used for watering the garden X 0.086 = £78.94

Flushing the Toilet

An old toilet system uses about 9 litres when flushed, inserting a South West Water hippo into the cistern would save around 3 litres per flush. 3 litres X 4 (visits to the toilet a day) = 12 litres X 4 (average family) = 48 X 365 (days in the year) = 17,520 litres of water which could be save in a year X 0.086 = £15.06

Do the Dishes by Hand

A dish washer can use up to 50 litres of water per wash.
50 litres X 365 (days in the year) = 18,250 litres of water used a year. Doing the dishes by hand uses 10 litres X 2 (twice a day) = 20 litres per day X 365 (days in the year) = 7,300 litres of water. By alternating between using a dish washer one day and doing the dishes by hand the next savings can be made. 18,250 / 2 = 9,125 litres of water required to use the dish washer every other day. 7,300 / 2 = 3,650 litres of water used by doing the dishes by hand every other day. 9,125 + 3,650 = 12,775 litres of water used by alternating dish washer use and doing the dishes by hand. 18,250 – 12,775 = 5,475 litres of water saved in a year X 0.086 = £4.70

Leave the Tap on whilst Brushing Your Teeth

A running tap flows at 15 litres per minute, if brushing teeth takes 2 minutes each time and it is done twice a day by four people that’s 15 X 2 = 30 X 2 = 60 X 4 = 240 litres per day X 365 = 87,600 litres per year. If you use a glass of water to rinse your mouth that’s approximately 0.5 litre per brushing X 2 times a day = 1 litre X 4 (average family) = 4 litres X 365 = 1,460 litres of water used per year by a family of four using a glass of water when brushing their teeth. 87,600 – 1, 460 = 86,140 litres of water saved each year X 0.086 = £74.08

Let the Tap run Cold for a Drink

Based on each member of an average family of four drinking 2 litres of water per day this totals 8 litres per day which is 32 drinks of 0.25 litre. Every time a glass is filled, 1 litre of water is wasted in letting the tap run cold. This is based on a tap flowing at 15 litres per minute and assuming that it takes only 4 seconds for the tap to run cold. 1 litre of water wasted X 32 drinks per day = 32 litres X 365 = 11,680 litres per year saved by keeping water in a bottle in the fridge instead of running the tap until cold X 0.086p = £10.04.

Use a Garden Sprinkler

A sprinkler uses 15 litres per minute X 60 = 900 litres per hour X 6 hours per week = 5,400 litres per week X 12 weeks of watering = 64,800 litres of water used X 0.086 = £55.72

Dripping Taps

A dripping tap can waste around 90 litres of water per week X 52 = 4,680 litres of water wasted per year X 0.086 = £4.02

Shower instead of Bath

A full bath holds around 150 litres of water X 4 (average family) = 600 litres of water X 365 (days in the year) = 219,000 litres of water used through bathing in a year for a family of four. A shower runs at an average rate of 6 litres per minute, therefore a 10 minute shower uses 60 litres of water X 4 (people in the family) = 240 litres per day X 365 = 87,600 litres per year. 219,000 – 87,600 = 131,400 litres of water saved through showering instead of bathing in a year X 0.086 = £113.00
This is an average figure, flow rates vary according to showers, a power shower would obviously use a lot more water.
The water used in a shower could also be re-used in a similar way to bath water.

Washing the Car

Washing the car uses 150 litres of water each time, if it is washed every week that’s 150 X 52 = 7,800 litres of water over a year. By washing the car every other week instead, 3,900 litres of water is saved annually X 0.086 = £3.35

Washing Machine

Washing machines use in the region of 150 litres of water for every load. By making sure only full loads are washed you could potentially save two loads each week.
150 litres X 2 = 300 litres per week X 52 weeks in the year = 15,600 litres of water saved a year X 0.086 = £13.41

Washing Face and Hands

A running tap uses 15 litres of water per minute. If the tap is running for 2 minutes whilst someone is washing their hands and face this is 15 X 2 = 30 litres of water used. 30 X 4 (average family) = 120 litres per day X 365 = 43, 800 litres are used per year. If a sink of water is used this is 5 litres each time X 4 (average family) = 20 litres per day X 365 = 7, 300 litres are used. 43,800 – 7,300 = 36,500 litres of water are saved by using a sink of water instead of leaving the tap to run. 36,500 X 0.086 = £31.39