Nobody really disputes that saving energy is a good thing — we pay less on our bills when we do, and cause fewer carbon emissions to boot. Getting people to cut back, though, has often proved pretty tricky. We like our comforts and routines. And, if a new study is to believed, we widely misperceive where the bulk of our energy use comes from — thinking that devices such as computers use much more energy than they actually use, even as we underestimate the contributions of major energy gluttons, such as home and water heating.

Why do we get it so wrong? The study in the Journal of Environmental Psychology has an intriguing answer: In general, people tend to incorrectly think that the objects that they use the most (such as computers and lights, which we’re constantly working on, or turning off and on) consume more energy than they actually do.

“Consumers tend to use the frequency with which they think or interact with a given energy source as a proxy for its consumption,” explains Ohio State University psychology doctoral student Dan Schley, lead author of the study (written with Ohio State’s Michael DeKay).

The hypothesized reason for this error is a version of the famous availability heuristic — a mental shortcut described by Nobel Prize-winning psychologist Daniel Kahneman (with Amos Tversky) that leads people to misperceive the likelihood of an event occurring because they’re relying on how easily it pops into their mind. An example might be fearing extremely rare shark attacks because you just watched the movie “Jaws.”

In the context of energy use, Schley and DeKay refer to a closely related concept that they call “cognitive accessibility.” The idea here is that a device that you use or think about regularly will be more cognitively accessible than one that you barely ever see or operate. And then you’ll think it uses more energy than it actually does.

The new study shows the power of this “cognitive accessibility” bias in four studies, in which people were asked to estimate what percentage of either their own home’s or the entire country’s residential energy use came from a variety of different sources — lighting, home heating, air conditioning, water heating and so on. At the same time, the research subjects — who included students, as well as adults from’s Mechanical Turk — were also asked “how often they interacted with devices in each category.”

According to 2005 data, home heating is the single greatest consumer of energy within homes across the nation (although vehicle driving, not always considered a “residential” form of energy use, takes up even more energy if it is included in the picture). Home heating is then followed by water heating, air conditioning and then lighting.

But the study found that people generally weren’t very good at estimating how much total energy use the different categories consumed. For one, they didn’t realize that the biggest energy users — home heating and driving “private motor vehicles” — were dramatically more energy intensive than many other smaller energy users, such as computers or dishwashers. Rather, while research subjects did generally know which the bigger and smaller energy users were, they underestimated the gap between them by a good bit.

“People tend to be insufficiently sensitive across the board,” says Schley. “They greatly underestimate the difference between dishwashers and cars.”

At the same time, people also underestimated, in an absolute sense, how much the really big energy users consume. For instance, in one of the studies, people estimated that home heating took up seven percent of energy use, whereas the actual answer is closer to 20 percent.

And there was a pattern in people’s misperceptions, one related to cognitive accessibility. The research subjects overestimated the amount of energy used by computers, cooking and televisions — all highly “accessible” object categories. At the same time, they low-balled the consumption of water heating — which tends to be less at the front of people’s minds — and small electric appliances that are used intermittently, such as coffee makers and hair dryers.

“These results are consistent with the substitution of a simple, readily accessible judgment (the frequency of one’s recent use of and thoughts about various devices) for a more difficult judgment (the percentage of annual energy used for a particular purpose),” wrote Schley and DeKay.

One particularly striking example of this phenomenon, as Schley explained by e-mail, is the constant focus on turning your lights off:

For example, consumers have long thought that turning-off the lights is the most effective strategy for curbing their electricity consumption. Our article suggests that part of the reason consumers believe this might be because consumers are constantly turning lights on and off. Because they use the lights a lot, they tend to infer that lights consume a lot of energy. On the other hand, consumers tend not to think about their water heating (other than when they run out of hot water) or interact with their water heater very often, as a consequence they tend to relatively underestimate the consumption of their water heater.

Not that you should leave lights on, of course, but the point is that lighting overall is just the fourth-largest consumer of residential energy, and individual lights consume far less than that. So if you really want to cut your energy use, then making sure to turn lights off is just the beginning.

Tallying up the sources of home energy use, and comparing them to people’s perceptions, is no idle exercise. For if we misperceive how we use energy, and how much of it we use in certain tasks or end uses, then we may also cut back in the wrong places – being satisfied, for instance, with unplugging a lot of electronics or turning off a lot of lights, rather than replacing a highly inefficient old water heater or HVAC system, or insulating or weatherizing a home.

What can be done? First, objects that we fail to pay adequate attention to could be made more salient to us. Schley suggests that people might have “a light outside their water closet, where their water heater is, that turns on and lets you know when your water heater is running, because it’s obviously running throughout the day to keep it warmed. Doing something like that would increase the accessibility for instance of a water heater, and by increasing its accessibility, it will increase its perceived consumption.”

More generally, Schley says, the more we move into an era of “smart homes” — where the hope is that someday, we’ll have devices that “disaggregate” our total energy use so that we can see which appliances and devices are using what — the less we may have this problem. “As the homes get smarter, they should ideally be able to see, ‘Oh, my refrigerator is taking up a ton of energy,’ and that’s what gets them to get a new refrigerator,” says Schley.

In the meantime, the practical advice here is pretty clear. Definitely turn off your lights and your computer, but don’t stop there. You should also check your insulation, and keep an eye on how you’re using hot water.