Data & ResearchMarch 12, 2008

Cooling Off Periods?

Conventional wisdom has long held that daylight saving time saves energy, but a new study suggests otherwise.

Reported by Nevin E. Adams

In fact, recent research conducted in Indiana found that residential electricity usage increased between 1% and 4%, some $8.6 million a year. It also said that “social costs’ from increased emissions were estimated at between $1.6 million and $5.3 million per year.

Up until two years ago, only 15 of Indiana’s 92 counties set their clocks an hour ahead in the spring and an hour back in the fall, with the remainder continuing on standard time all year (at least in part because farmers in the Hoosier State resisted the prospect of having to work an extra hour in the morning dark). That finally came to an end after the Indiana Legislature voted to put the entire state on daylight-saving time beginning in the spring of 2006.

That change provided University of California-Santa Barbara economics professor Matthew Kotchen and Ph.D. student Laura Grant a unique way to see how the time shift affects energy use (see “Does Daylight Saving Time Save Energy? Evidence from a Natural Experiment in Indiana’). According to Reuters, they were able to compare energy consumption before and after counties began observing daylight-saving time (readings from counties that had already adopted daylight-saving time provided a control group that helped them to adjust for changes in weather from one year to the next).

Their finding: Having the entire state switch to daylight-saving time each year costs Indiana households an additional $8.6 million in electricity bills. The researchers concluded that the reduced cost of lighting in afternoons during daylight-saving time is more than offset by the higher air-conditioning costs on hot afternoons and increased heating costs on cool mornings.

DST Origins

Daylight Saving Time (DST) was not formally adopted in the U.S. until 1918, as part of the establishment of standard time zones (Ben Franklin is widely credited with the notion, expressed in his 1784 “Essay on Daylight Saving’.

DST was observed for seven months in 1918 and 1919, but after World War I ended, the law proved so unpopular (mostly because people rose earlier and went to bed earlier than today), it was repealed in 1919 with a Congressional override of President Woodrow Wilson’s veto. Daylight Saving Time – note that it is not savings time – became a local option (for more information, see A Stitch in Time). The Uniform Time Act, passed in 1966, established a system of uniform (within each time zone) daylight savings time throughout the U.S. and its possessions, except where legislatures voted to keep the entire state on standard time (these days that is Hawaii, American Samoa, Guam, Puerto Rico, the Virgin Islands, and in most of Arizona – the exception being the Navajo Indian Reservation there, which does recognize DST).

Over time, most states began changing their clocks, and in response to the 1973 oil shock, the country extended daylight-saving time in 1974 and 1975. Analyzing that time shift, a 1975 report by the U.S. Department of Transportation concluded that the change reduced electricity demand by 1% in March and April, according to Reuters (less well-reported was a 1976 report to Congress by the National Bureau of Standards that concluded that there were no significant energy savings from the move).

Air “Conditioning”

However, the energy-savings numbers often cited by lawmakers and others come from research conducted in the 1970s – which, according to researchers, overlooks the prevalence of air conditioning today. The most recent study showed that while an extra hour of daylight in the evenings may mean less electricity is spent on lights, it also means that houses are warmer in the summer when people come home from work. Conversely, during daylight-saving time’s cooler months, people may crank up the thermostats more in the morning.

The results might be different in states with a different geography than Indiana, or course. And there might also be social benefits to daylight-saving time not addressed in the research (other studies have noted “less crime, fewer traffic fatalities, more recreation time and increased economic activity” with the extra sunlight in the evening). But then there’s that impact on your circadian rhythm (see A Stitch in Time).

The most recent study is available online at http://www2.bren.ucsb.edu/~kotchen/links/DSTpaper.pdf