Influence of Resident Behavior on Energy Consumption

Residential energy behaviour plays a particularly important role in determining real energy consumption for buildings with a high potential of passive solar energy utilisation. However, this factor is usually taken into account only as a vague estimate in planning and construction. The aim of the project was therefore to determine the “human factor” in energy-saving potential more closely, and to create the basis for strategies to make better use of this potential.

The low-energy housing development “Solar Housing Plabutsch” in Graz, Styria, was built by the housing co-operative Neue Heimat; its design by Architect Adil Lari was awarded the Prize of the State of Austria for Housing Construction, and is subject to the criteria of subsidised housing. To investigate the impact of residential energy use on actual energy needs, the building was assessed by means of empirical measurements over a period of two and a half years.

Besides recording the surrounding temperature and wind speed, heat consumption was measured by electronic heat costs distributors for all apartments. In addition, heat meters were installed in six apartments; three of these apartments were equipped with window contacts, as well as potentiometers to record the position of the blinds. Furthermore, a survey was carried out among the residents to assess their attitude towards ecology and the building itself, as well as their willingness and their strategies to save energy.

Due to measures taken in planning and construction, the building’s heating energy needs are calculated at 32 kWh/m²a according to simulations. This value, which corresponds to the standard for low-energy buildings, refers to the building alone, without taking into account the residents. Depending on building utilisation and in-home energy use, heating energy needs can be reduced to as little as 11.5 kWh/m²a, meeting standards for passive solar housing.

Average heating energy consumption was measured at 56.24 kWh/m²a for the year 2000, i.e. 76% higher than the calculated value. In the period from 20 July 2000 to 22 June 2001, average heating energy consumption dropped to 40.97 kWh/m²a (28% above the calculated value), possibly due to the fact that trapped humidity had diminished due to heating, or that the residents had learned to make better use of the building’s inherent energy-saving potential. Exact measurements in individual apartments pointed to a deviation of 50%–211% from the calculated value, indicating that the energy-saving potential of the building was not fully utilised by the residents.

The residents were informed about strategies to utilise the building’s energy-saving potential by means of a user manual distributed during an information event. Among the recommended strategies was shock airing, i.e. fully opening all windows and connecting doors for five minutes every two hours, so as to achieve a thorough circulation of fresh air. The blinds should not only be used to shade against sunlight, but also closed at night so as to prevent heat losses to the outside environment. Indoor temperature should be kept constant at 20-21°C during the day, as heating energy needs rise disproportionately with higher temperatures.

The survey among the building’s residents (52% of questionnaires returned) clearly showed that the residents were prepared to save energy and greatly interested in ecology. 50% named the wish to reduce energy consumption and live in an environment-friendly building as a “very important” motive for moving into the building. 25% considered these factors “rather important”.

All those interviewed agreed that in order to actually save energy in low-energy buildings, residents have to act in an energy-conserving manner. 85% were “very” or “rather” convinced that it is comfortable not to have to worry about wasting energy because “the electro-mechanical facilities take care of saving energy”. 94% were “very” or “rather” convinced that the ecological design of the building makes it easy to live in an ecological way. Only 39% said that they paid increased attention to saving energy. The results indicate a certain rebound effect.

A comparison of the answers to the survey and actual resident habits (in periods from December to April) in three empirically measured apartments points to numerous discrepancies. For instance, while most of those surveyed said that they assured fresh air supply in winter by opening their windows several times a day for up to 15 minutes, windows remained open or half-open mainly over longer periods of time in the measured apartments. Asked for the desired room temperature in winter, the residents stated 18-22°C during the day and 16-21°C at night, depending on room type. They estimated their actual room temperatures at 18-20°C for daytime and at 16-20°C during the night. In fact, average room temperatures between 21°C and 26°C were recorded in the measured apartments.

Keeping all other factors constant, simulations were carried out for the empirically measured apartments based on the assumption of optimised resident habits so as to investigate their effect. If room temperatures were maintained according to standard (and to the desired levels stated in the survey), i.e. 20°C during the day and a reduction to 17°C at night (meaning an actual room temperature of 19°C due to stored heat), heating energy needs would decrease by an average 60%. Standardised airing habits would result in an average 48% decrease in heating energy needs.

Optimised resident habits can therefore lead to a considerable decrease in heating energy needs, and consequently a decrease in operating costs. If all residents of the building lowered their energy needs to the level calculated in the simulation based on standardised in-home energy use, total costs would be reduced by 30%. Due to the 35% basic cost component which depends on apartment size, cost reduction potential is lower if only individual residents save energy: in this case, a 30% reduction in energy needs results in a 24% cost reduction.

Since the residents’ will to save energy is documented, information about optimum utilisation of the building, which was provided in this case by means of an information event and user manuals, apparently has yet to be intensified. Given optimum utilisation, the building has heating costs around one third lower than those of comparable buildings. With framework conditions unchanged, low-energy residential buildings are therefore not only feasible, but also make economic sense, both for the building owner and the residents. Extrapolated to energy policy in general, this means that with all other factors kept constant, a considerable energy-saving potential can be accessed merely by influencing residential energy use.