What is sustainability – we need to use energy per person not energy per floor area for houses

20130325-221239.jpg When the problem becomes harder and the number of solutions are multiple, the possible answers become more emotive. In a recent discussion about achieving zero waste in the office the topic of alternatives to throw away coffee cups came up, the discussion became very emotive as the some of the possible answers affected our lifestyles.

One of the reasons why I love tools like Eco-footprint and the Impact Equation is that they use empirical evidence to neutralise the emotive responses! It can be used to say whether not using throw away coffee cups is better or worse that riding a motorbike.

We also sometimes trip ourselves up by using conflicting parameter for assessing our environmental footprint, a recent example was the use of carbon as a proxy in NABERS ratings.

As another example, following on from my post about energy consumption in houses we need to be careful about using the right parameter for assessing the energy performance of our houses.

Space Heating in Houses
Over the last 10 years in Australia we have quite rightly focused on improving the energy efficiency of our new homes through energy rating regulation. This focus has produced an excellent reduction in the amount of space heating energy on a per m2 of house floor area basis – some 36% reduction in energy from all houses new and existing.


But, in the same period we have also seen a 35% increase in the average floor area per house without an increase in the number of people per household. So our houses are larger, they take less energy per m2 of floor area to heat but we have more floor area per person.

The net result is that we have only seen an 8% reduction in space heating per person since 1990!

Now, if a house is empty there is no space heating demand, if there is one person per house rather than 4 the house consumes the same amount of energy regardless of the number of people so we shouldn’t use energy per floor area as our measure we should use energy per person.

Our impact after all is equal to number of people times service per person, not the number of houses. When we use per meter squared of floor area as a measure of performance we lose the impact of how much floor area each person has.

Appliance Energy in Houses

If we were to use the same calculation for appliance energy consumption, our appliance energy use per m2 floor area has only increased by 3% between 1990 and 2010.


But, our appliance energy consumption per person has increased by 48% in the same period.

The measure of appliance energy per person is a lot more reflective of the energy impact than appliance energy per house or per meter squared of floor area.

Per person, not per m2

The strong improvements in building envelope performance (36%) have been pretty much wiped out by the 35% increase in average house size, the measure of energy efficiency performance per meter squared has disguised this. Our space energy per person has as near a damn it stayed flat for the last 20 years.

When you take into account the amount of floor area per person now versus a 1950s house, I would postulate that a poor energy rated 1950s house would perform better in energy per person terms than a new 6* Energy rated house that is some 40-50% bigger!

When we start to compare one thing to another we need to make sure we pick the right parameters to compare then against.

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