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Quantifying the Energy and Carbon Effects of Water Saving (2009)

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This report by the Environment Agency and Saving Energy Trust seeks to determine the CO2 emissions resulting from specific water using activities and behaviours in the home. Building on a 2008 study, it calculates the CO2 quantities generated at different stages of the water supply-use-treatment process. It also estimates which household appliances produce how much CO2 emissions, considering hot and cold water use, different energy sources, and use scenarios between existing and new houses. Additionally, the study provides numbers about financial costs of CO2 savings from various measures has been quantified.


The overall CO2 emissions from the UK water industry are relatively well known and understood. However, a recent Environment Agency Study (2008a) has demonstrated that the major greenhouse gas emissions associated with the supply-use-treatment cycle of water use in the domestic sector are during the ‘use’ phase of water with 89% of emissions attributable to water use in the home (with the remaining 11% attributable to utility companies). It is therefore important to understand the CO2emissions from specific water using activities and behaviours in the home, and this is what the current study addresses. Two approaches have been used in the current study. The first simple Water Energy Model (WEMlite) calculates hot water use or saving at the appliance and then divides by boiler efficiency to determine the fuel and therefore carbon used. However, in some instances this approach is too simplistic, because there are ‘fixed’ losses due to the hot water system (such as heat loss from the cylinder and pipe work), and during the heating season some of these heat losses could be considered to be useful gains. A second model (WEM, Water Energy model) incorporates these system losses and can therefore be used to evaluate improvements in cylinder and pipe insulation and plumbing design as well as the effects of changing volumes of water used.

Both models include an estimate of utility company CO2 emissions and these can be added to each water-using component. To provide context, these emissions are set out in Figure 1 for a typical new build home. The proportion attributable to the utility company compared to domestic emissions is very close to the 11% calculated in the Environment Agency study (Environment Agency, 2008a).

The output from the Water Energy model in terms of CO2 emissions from a standard existing dwelling with a gas system boiler (78% efficiency), hot water cylinder (120 litres with 25mm foam insulation) and standard water use are shown in Figure 2.
As expected, appliances using hot water dominate. Also, since heating water using electricity results in higher CO2 emissions than heating the same volume of wate using gas, the CO2 emissions associated with white goods are proportionally higher than for the other hot water uses. It is therefore very important to consider the differences in CO2 emissions between different fuel sources (i.e. the fuel factor) when calculating the impacts of measures (the CO2 emissions from an identical water use in a home using entirely electrical heating will be approximately double that in a home in which a gas boiler provides hot water). It is also important to recognise that the results are very sensitive to assumptions about what we regard as standard water use.