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One would assume that using less of something every time you need to use it would mean that you use less of it overall, right?

Alas, it's not that simple. It seems we cannot take efficiency gains for granted, particularly with respect to energy where we most need such gains. If you use less petrol per mile, perhaps you travel further. If you save money on your domestic energy bills, perhaps you spend it on a foreign holiday.

This crude rendering of a complex idea is known as 'the rebound effect', which is a controversial issue in certain circles.

Some say we drastically underestimate how big it is, and therefore squander resources in trying to improve efficiency; gains that are later wiped out because we don't address underlying causes relating to attitudes and values. Others says we drastically overestimate the rebound effect and undervalue and fail to prioritise perfectly good and tangible environmental gains (e.g. cavity wall or loft installation) for fear of rebounds. My own view is that the effect is likely to be quite large, but in most cases energy efficiency gains are still well worth pursuing. It seems to make sense to start on the relatively easy target of energy waste before moving on to the much tougher target of energy use.

the idea is not just that increasing energy efficiency doesn't always save that much energy, but that, perversely, it actually leads to more energy being used.

The question of how big the effect is is ultimately empirical in nature, but extremely hard to measure. Clearly it varies depending on the product and the activity. Efficiency gains in fridges are likely to be absolute for instance, because they are on all the time anyway, while efficiency gains in lights are not so clear, because you may feel less bothered to turn them off.

A couple of years ago we built a whole project around the fuel efficient driving of taxi drivers, because we believed we might learn important things about behaviour change as a result. I think we did, but my strong impression is that the drivers were motivated by cash savings rather than any environmental benefit of those changes, which at least begs the question of how much embodied carbon the chosen product or service they buy with the money saved will have.

Anyway, the real purpose of this short blog was to introduce you to one version of the rebound effect, known as the Khazzoom-Brookes Postulate, which is rather extreme, and slightly amusing, at least partly because of the name. In this case the idea is not just that increasing energy efficiency doesn't always save that much energy, but that, perversely, it actually leads to more energy being used.

The simple expression of the postulate is: "energy efficiency improvements that, on the broadest considerations, are economically justified at the microlevel, lead to higher levels of energy consumption at the macrolevel." Needless to say this postulate is not universally accepted as being true to reality, which is probably why it's still called a 'postulate'.

I mention this now in response to a tweet message from Nick Stanhope, The CEO of 'We are what we do' who kindly forwarded an article in Scientific American suggesting that, at least in the US, the rebound effect has been shown to be small. In fact, the article quotes a few experts with that point of view, with no real supporting evidence, so to my mind the key questions remain:

How big is the rebound effect? How might we find out? And can you say Khazzoom-Brookes with a straight face?


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