The prebound effect

Many European households are consuming less energy than predicted, especially in supposedly energy-inefficient homes, a new study has found.

The research identified a recurring gulf between the quantity of energy predicted by governments for different types of housing and the amount homeowners actually use.

Researchers also found that the discrepancy was greatest among the least energy-efficient homes, where householders appear to be consuming far less than national energy usage standards predict.

This phenomenon is branded the “prebound effect” in the study, which is published in the new issue of the journal Building Research and Information. (The term refers to the earlier identification of a “rebound effect”, in which people who have already had energy-saving initiatives such as thermal retrofits implemented in their home then use more energy, reducing the amount of energy actually saved).

Conversely, the “prebound effect” suggests that politicians and policy-makers who want to see more such initiatives in a wider range of homes may be over-estimating the benefits, and the rate of pay-back, because their judgements about how much energy those homes consume are already exaggerated.

Dr Minna Sunikka-Blank, from the Department of Architecture at the University of Cambridge, who co-authored the study, said: “In general, the worse a home is thermally, the more the occupants tend to control the amount of heating they use. For financial reasons, they also have to. This challenges the prevailing view that large cuts in energy consumption can be achieved by focusing purely on technical solutions, such as retrofitting homes. In some cases, doing so may bring only half the expected savings, perhaps less.”

The study focused on data from Germany, although it then found similar patterns in several other European countries including the United Kingdom. Germany has a rigorous thermal retrofitting programme which has been seen as a leading model for other European states.

At the heart of this model is the use of the Energiekennwert, or energy performance rating (EPR), which is a figure used by German policy-makers to predict the energy consumption of a given type of dwelling based on the thermal quality of the building, the heating system and the location. This is used to predict not only the amount of energy consumed, but the amount that might be saved with improved insulation, for example, as the result of a thermal retrofit.

Sunikka-Blank and her colleague, Dr Ray Galvin, used EPR and energy use data for 3,400 dwellings in Germany, to model the difference between predicted energy consumption and the amount of energy actually being consumed. Most of the measured figures came from meters in people’s homes. In addition, the research drew on background data about the physical character and energy consumption of a further 1 million properties.

The model revealed clear discrepancies between calculated and measured energy consumption. Even when comparing homes that fell into the same predicted energy bracket, cases were found where one house used six times as much energy as another.

Critically, however, the study revealed a gulf between the predicted energy consumption for heating and the amount actually measured. The average EPR for a German dwelling was about 225 kilowatt hours per square metre, per year (kWh/m²a). Real energy consumption for heating averaged at around 150 kWh/m²a; a discrepancy of 30%.

When the discrepancies were plotted on a graph, they showed that overall, the higher the EPR (and therefore the lower the energy-efficiency of a house), the lower the relative measured energy consumption turned out to be. For example, the real average energy consumption of a home with an EPR of 300 kWh/m²a was about 40% below the calculated value, whereas that of a home with an EPR of 150 kWh/m²a was only 17% lower.

In practice, this means that many homes which are predicted to be highly inefficient in terms of the amount of energy they consume, are often consuming nowhere near as much energy in practice.

Comparison with further data, assembled by other researchers in different studies, revealed that similar patterns can be seen with homes in the United Kingdom, the Netherlands, Belgium and France.

In the UK, energy efficiency is predicted using a measured called the Standard Assessment Procedure (SAP), on a scale of 1-100, where 100 is the most efficient. Again, homes with a low SAP have been found to consume far less energy relative to their rating than those with a high SAP. As with the German example, the higher the predicted energy consumption, the lower the actual relative energy consumption seems to be.

The study suggests that measures such as the German EPR may be based on flawed assumptions about important factors such as energy loss through ventilation, or standard indoor temperature. There may also be a discrepancy between the ways in which buildings are designed and how they are built in practice.

Fundamentally, however, the study indicates that predictive measures are failing to take into account the ways in which people actually heat their homes in practice. “It seems that many German households tend to keep their homes cooler, or heat fewer rooms in their home, or have their heating on for less time - or various combinations of these - than is assumed in their EPR calculations,” Sunikka-Blank said.

“As retrofits cannot save energy that is not actually being consumed, this has implications for the economic viability of thermal retrofits.”

The study adds that further research is needed to explain the prebound effect, but hints that one reason may be budget-consciousness among families living in energy inefficient homes. This appeared to be borne out by interviews the researchers conducted with German householders.

The full paper will be downloadable for free for a limited time from the journal website, http://dx.doi.org/10.1080/09613218.2012.690952