Is There a Case for Extending Daylight Saving Time?

It’s that time of year again, for North America at least. As we turned back our clocks on November 4th, the prospect of long, dark, and dreary winter evenings caused many of us to ask once again – why is it exactly that we use Daylight Saving Time (DST)?

It seems that ever since this practice began, debate has raged on the potential benefits and disadvantages of the bi-annual ritual. We increasingly hear arguments advocating for extending DST year-round, mostly on an environmental basis: if DST saves energy, why not make it permanent, and thereby reduce national GHG emissions?

The paper “Is There a Case for Extending Daylight Saving Time” takes a closer look at this reasoning. We examine how the complex interactions between electricity use and daylight hours makes it challenging to determine electricity savings from year round DST, and explain why the applied energy mix is in fact the more important factor in terms of GHG emissions.

The Energy Policy Act of 2005, which went into effect in 2007, extended daylight saving time in the U.S. by four weeks. The U.S. Department of Energy (USDOE) took advantage of this unique event to measure the impact on electricity use across the country, and determined that 0.5% of electricity consumption was saved for each day of extended DST.

We did a quick calculation assuming average savings of 0.5% per day and found that extending DST from November 4, 2007 to March 9, 2008 roughly results in a reduction of 6305 million kWh of electricity and 3,716,524 tCO2e of avoided emissions.

This assumption however is highly flawed. Electricity use varies when daylight is extended during waking hours because of:

• Lighting – more daylight means not as many lights are turned on
• Heating and cooling – more daylight means less heating in cool weather but more cooling in warm weather
• Outdoor recreational activity – more daylight means more time is spent outside not using electricity.

The extent to which each of these factors affects electricity use changes depending on geographic location, time of day and time of year. The complexity of these interactions, not sufficiently examined in the USDOE study, implies that an assumption of 0.5% daily electricity reduction cannot be applied to assess year round DST savings.

However, what we are really interested in examining is the impact year round DST would have on GHG emissions, and how this is tied to a region’s energy mix. By comparing the electricity savings from the four week extension in Florida (60 GWh) and New England (68 GWh), we find that realized GHG savings were greater in Florida than in New England (33,358 tCO2e compared with 25,737 tCO2e, respectively), because of the higher emissions associated with Florida’s applied energy mix (for example, around 27% of power generation comes from coal in Florida compared with 15% in New England).

Thus, any savings in electricity use gained by extending DST can be easily eclipsed by the composition of energy sources used to generate the electricity being consumed. If we want to get serious about policy changes that will reduce GHG emissions, changes that affect how we produce our energy would have a much greater impact than changing the clocks.

Reading Time: 4 Minutes

Date Published: December 9, 2012



It’s that time of year […]


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