Potable Water Usage

Our water meter was installed by Welsh Water (Dŵr Cymru) on 18th March 2020. Time for a quick update to see whether we are frugal or wasteful users of potable water.

First of all, let's establish a baseline water usage for UK households. Statista reports the average daily water use per person at around 142 litres; this does not include water used outside the house for watering the garden, washing the car, etc. Other sources suggest similar per-person values for the UK. Welsh Water are promising to add a feature to their website allowing comparison with other customers' water usage.

Surprisingly, most people haven't got a clue how much water they use; nearly 50% of Britons think they use under 20 litres per day.

Showers and toilets account for about half the daily usage. I couldn't believe it when the Guardian reported the average shower takes seven and a half minutes!? When I timed myself this morning, it was longer than a minute but shorter than 2. I was fairly confident we would come under the UK average for daily water use especially considering we also follow the old maxim about toilet flushing: if it's yellow let it mellow, if it's brown flush it down.

After installation of the water meter, I took 'monthly' readings to get a baseline indication of water usage and check for possible leakage. The last 3 measurements are the six-monthly readings of the water company for billing purposes. 

DATE OF READING	NUMBER OF DAYS	WATER METER READING (㎥)
18/3/2020	0	0
18/4/2020	31	8
18/5/2020	61	20
26/5/2020	69	28
19/6/2020	93	32
18/7/2020	122	42
19/9/2020	185	59
16/11/2020	243	73
17/5/2021	425	121
30/11/2021	623	199

Plotting water consumption as a time series indicates a fairly uniform rate of use over the 20 months of data. Linear regression shows an average household daily rate of 307 litres (0.307 ㎥) or 153.5 litres per person per day. [Note: 1 ㎥ = 1000 litres]

At first glance, this was a somewhat disappointing result when compared with the UK average of 142 litres per person per day. However, there are two confounding factors. Firstly, for a three month period in 2021 (June-July-August) the household consisted of four people rather than two. Second, the UK average does not include water used outside the home - in our case this was mainly garden watering.

To account for the change in household numbers for the 3-month (90 day) period, I guesstimate our two extra guests each used 100 litres/day - so 200 litres/day in total - because they were not around all the time. This amends the water usage plot above to that below and a new average of 142 litres/person for our 2-person household, in line with the UK average. Our average also includes garden watering which the UK average does not.

Estimating the amount of water used in the garden is difficult. The excellent straight line correlation in the plots above suggest fairly uniform use throughout the year which would imply limited potable water use for irrigation. The last 3 readings (see Table) cover a Winter/Spring (16/11/20 to 17/5/21) and a Summer/Autumn (18/5/21 to 30/11/21) period. After allowing for the extra guests during the Summer, average daily water use went from 264 litres/day in Winter/Spring to 303 litres/day in Summer/Autumn. Hence, about 50 litres/day, on average, during the growing season (May to September) of which roughly 25 litres/day is used for the Quadgrows in the Polytunnel.

Is 50 litres/day a big number or a small number? This website from the University of California suggests a 3 square metre plot needs 90 litres/week in summer. So my vegetable plot (100 square metres) requires 430 litres/day. Some of this will come from rainfall, but it does confirm that 50 litres/day is a very efficient use of water. Especially when that 50 litres/day includes irrigation of the rest of the garden which is about another 150 square meters. Our waterbutts (about 3000 litres) undoubtedly help keep the total potable water use to a low level.