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.


  1. The DST debate in the UK actually centres around child safety, not saving energy. Consider that the UK is roughly the same latitude as southern Canada. The argument being that kids go to and from school in daylight. “RoSPA” web site suggests around 80 lives and 212 serious injuries are saved. How many would this scale up to in the US? Since the number of dark hours is the same and many people work flexible hours (ie. not 9-5), I question the 0.5% energy saving. Many of us go to work in the dark and come home in the dark, so where is the saving?

    1. Hi Simon, thanks for your comment. Indeed, you are quite right – the DST debate goes far beyond questions of energy usage. In fact, everything from impacts on road accidents (including the safety of children waiting for the bus in the morning), to health (from disturbed sleep patterns and increases in heart attacks, according to some studies), and even economic activity (by increasing tourism and outdoor recreational activity) have been cited as arguments both for and against extending DST. The 0.5% daily electricity savings is what was measured by the U.S. Department of Energy when DST was extended by 4 weeks in 2007. The factors behind these savings likely stem from reduced reliance on artificial light, less need for heating when DST is extended during cooler weather, and more time spent outdoors during warmer weather. However, the strength of each of these factors varies depending on the time of year and location. This probably explains why other studies that focused on smaller areas (such as a 2006 study in Indiana which found a 1% increase in residential electricity demand when the state adopted DST for the first time – likely because of higher air conditioning needs) come to different conclusions. I focused on the USDOE study because it is the most thorough and complete analysis I could find. Either way, from an environmental standpoint, a region’s applied energy mix still has much larger impact on GHG emissions than whatever electricity savings extending DST could – arguably – bear.

  2. Bonjour et Merci pour remettre cette question à l’ordre du jour.
    Doit-on réduire une telle mesure qu’à un calcul comptable de GES ? Toute électricité réorientée vers une meilleure utilisation n’est-elle pas un gain ? même au Québec où l’électricité est sans GES. Votre analyse tient-elle compte de l’impact extrêmement positif d’une heure de plus de clarté en hiver pour les populations du nord ? Impact en termes de productivité, de bien être psychologique et de qualité de vie. Pour l’air climatisée, quelle différence y aurait-il puisque nous sommes déjà en DSL en été ?

    1. Merci Benoit pour votre commentaire. Le débat sur l’heure avancée repose en effet sur plusieurs enjeux – ce n’est pas seulement une question d’impact sur la consommation d’énergie. Les défenseurs de chaque camp soulignent plusieurs avantages et désavantages, comme je l’indique dans ma réplique ci-haut. Aux fins de cet article, j’ai choisi de me pencher exclusivement sur l’argument environnemental, en examinant l’impact que l’extension de l’heure avancée pourrait avoir sur les changements climatiques. Par rapport à votre question sur l’air climatisé, c’est un bon exemple du rôle important que jouent la géographie et le temps de l’année sur l’influence relative de chaque facteur: au Québec en janvier, vous avez raison, le niveau d’utilisation d’air climatisé n’aura pas un rôle important. Mais en Floride au mois de mars, ce serait tout à fait possible.

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