Leave the Lights On or Turn them Off?
In 1988 and 1989, there was an interesting and humorous sequence of papers in Interfaces considering whether or not lights should be turned off when leaving a room. The primary trade-off is that while energy is saved with the bulbs off, the bulb life span is diminished each time it is turned on.
The first article, Should We Turn off the Lights When Leaving? by Charles Mosier, George Sheldon and Glen Avery (full citation below) proposes a simple decision rule for turning off lights when leaving a room that depends on:
- arrival rate of individuals into the room
- amount of time an individual is in a room
- power usage per bulb (in watts)
- electricity cost ($ per kilowatt hour)
- purchase price of a bulb
- labor cost of replacing a bulb
A few months later, the article Turn off the Lights by Paul Randolph appeared in Interfaces. Randolph questions extrapolations of a linear regression used in the Mosier et al. paper to relate bulb life span to maximum starts per hour. He also points out that determination of arrival rate and time spent in a room would require “considerable data collection”, suggesting this would be more work than is justified. But essentially, he argues that bulb life reductions due to start-ups are minimal with the technology of that day (late 1980s) and are more than compensated by the power savings. Along the way he cites his academic vice president, Ann Landers, and a Navy Civil Engineering Laboratory. And this left Mosier and Sheldon with a huge opening. In Leave the D*** Lights On, they start by picking apart these sources:
…it is common knowledge among experienced faculty that academic vice-presidents and their ilk are simply not to be cited in cases where truth and accuracy are considered important.
Ann Landers is taken to task for her “lack of skill in applying scientific thought to a problem” as illustrated in another column of hers on weight loss product advertisements. And the Navy, while credited for being good at what they do, is questioned as a model for least cost operation. Mosier and Sheldon get serious later on in the paper, addressing the issues raised by Randolph. See the papers for more information.
The technology has changed even more since the time the papers were written and light sensors have become far more wide-spread. But the debate continues. The US Dept. of Energy has a web page about the topic. It says:
the operating life of all types of light bulbs is affected by how many times they are turned on and off. The more often they are switched on and off, the lower their operating life. The exact number of hours that switching lights on and off reduces the total operating life depends on the type of light and how many times it is switched on and off.
And about fluorescent bulbs:
For most areas of the United States, a general rule-of-thumb for when to turn off a fluorescent light is if you leave a room for more than 15 minutes, it is probably more cost effective to turn the light off. Or in other words, if you leave the room for only up to 15 minutes, it will generally be more cost effective to leave the light(s) on. In areas where electric rates are high and/or during peak demand periods, this period may be as low as 5 minutes.
The citations for the papers discussed above are:
Should We Turn off the Lights When Leaving?
by Charles T. Mosier, George W. Sheldon and Glen E. Avery
in Interfaces, Vol. 18, No. 5 (Sep. – Oct., 1988), pp. 62-71
Turn off the Lights
by Paul H. Randolph
in Interfaces, Vol. 19, No. 4 (Jul. – Aug., 1989), pp. 89-91
Leave the D Lights On
by Charles T. Mosier and George Sheldon in Interfaces, Vol. 19, No. 4 (Jul. – Aug., 1989), pp. 92-95
Update – just found this related information: http://www.naturalhandyman.com/iip/infelectrical/inflightsoff.html