Tuesday, 8 July 2014

What is normal energy use?

Households vary enormously as to how much energy they use and what it is for. This is the clearest lesson I have learned from my analysis of the Household Electricity Survey(HES) [1] and I see it in data sets for other sorts of energy too. I recently read a report about a 'Smart Community' project which aimed to reduce energy use in households recruited in Kingston upon Thames[2]. One of their findings was that when people discuss their energy use habits this seems to reinforce their current usage rather than promote savings. However, when I got to think about it this was not so surprising, and I suspect it leads to increasing energy use rather than savings, at least in some categories.

The graph below shows the varying pattern of washing machine use across all households from the Household Electricity Survey. The median is 4.2 times per week but the middle half range from 2.3 times/week to 6.5 times/week. That means a quarter of households use their machine less than 2.3 times per week and another quarter use it nearly three times as often. Before you say, oh it depends on the number of people in the household, considering only 2-person households (69 cases), the range for the middle half was almost as large - from 2.1 to 5.7 times per week.

The number of times households use their washing machine per week varies hugely - each line represents one household. From the HES [1]

Now suppose you are in a room with other people, some of them your neighbours and some others you know by sight. One person says they only use the washing machine once per day and they always use the eco setting. Now if you are one of the people who do washing only 2-3 times per week are you going to stand up and say this? Probably not because you don't want people to think you are 'dirtier' than average. That means that other people in the room who also wash quite often get the impression that using the washing machine every day is the norm and those who don't use it so often think maybe they should use it more.

There is huge variation in other behaviours too: for example using the shower, watching TV, boiling the kettle and so on - as shown in the table below. As for what we do in the shower, well a study in Australia found people shaving their legs, playing with toys and lots of other activities, all of which take time and use more water [3]. Whether British people are similar is an interesting question.

WhatMiddle half (25% to 75% range)
Running the washing machine2.3 - 6.5 times/week
Watching TV4.5 - 10.5 hours/day
Boiling the kettle18 - 48 times/week
Showers1.2 - 4.3 times/person/week
Data from the HES [1]

It isn't just electricity use either. An EST study measured hot water use in 112 households. The chart below shows the range they found broken down by household size - 4 person households used from 50 to 400 litres/day of hot water [4].

Hot water use as measured by the EST. Each dot is one household [4].

Appliances use energy and heating water uses energy. These differences imply big differences in energy use too. For the middle half of households the proportion of electricity used for laundry and the dishwasher varies from 5% to 15%, and for 5% of households this accounted for more than a quarter of the electricity bill.  If we are trying to reduce our energy use, how can we decide what is a reasonable level, based on this enormous variation? Do we ask our friends what they do,  and if we then do we trust them to be unbiased in their answer? Or do we not ask in case they will ask us back? Surveys like this cost a great deal to run, but they at least they give us unbiased information.

[1] Household Electricity Survey (DECC) 2014
[2] Smart Communities: Working together to save energy (Kingston Business School) 2014
[3] Measurement of hot water consumption in dwellings (EST) 2008
[4] Revealing look at shower habits (Daily Telegraph (Australia)) 2006

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