Saturday, 24 March 2012

Daylight saving and energy saving

Weekday electricity usage 2011 averaged over Mon/Tue/Wed around the clock change. Sunset time is for London.
We switch to Daylight Saving Time tomorrow (March 25th). This changes our electricity consumption patterns. The Lighter Later campaign would like us to switch to DST all the time and DST+1 in summer, for various reasons including a predicted reduction in electricity demand. Is this estimation reasonable?


You can see in the chart above how the clock change was reflected in our electricity consumption in 2011.
  • The evening peak demand occurs about an hour after sunset. The clock change change shifts this peak to an hour later.
  • Later peaks also tend to be lower. In the second week after the clock change, the evening peak is actually lower than the daytime peak which is just before lunch.
  • The week after the clock change we use less overall energy than the week before (by 0.2%).
The lighter later campaign refers to a study from the University of Cambridge Engineering Department which uses statistical analysis based on data from 2001 to 2008 to show that their proposed change in policy would result in:
  • Overall drop in electricity demand 0.3%
  • Reductions of up to 4.3% in peak demand, leading to lower electricity prices and reduced carbon emissions because less expensive 'peaking plant' is required (See also Smart meters will save us money for a discussion on why peaking is going to get more expensive in the future)
I dare say the statistical analysis is robust, but that doesn't mean that the predictions in this study are accurate. Technology changes and we know that the energy used for lighting is going to decrease because of increased use of energy saving light bulbs. Compact fluorescents use barely a third the power of incandenscents and LEDs use even less. In fact electricity used for lighting peaked around 2002 and is already decreasing albeit slowly. Over the period of the study, lighting energy use reduced but only by 3.5% [1].

It is very difficult to know how much of the predicted reduction in electricity use is due to lighting, and how much to behaviour differences. Maybe we use the extra daylight for gardening, or other outdoor leisure activities. Then when it gets too dark to tell the weeds from the flowers or to track a football we come inside and start cooking, or make a cup of tea and turn on the TV or our computer. In that case switching to DST all year will continue to reduce peak demand even when we all use LED light bulbs (and also be good for our fitness as we spend more time on outdoor activities). If on the other hand we are sitting inside anyway, and the only difference is whether or not we have the lights on then the energy saving advantage of DST will be largely temporary.


[1] Great Britains Housing Energy Factfile  (DECC)

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