The boxy air conditioners that fill so many urban windows must consume a tremendous amount of energy. From an environmental perspective, aren’t central air conditioners better?

Not necessarily. Air conditioning accounts for more than 15 percent of the energy use of the average home, consuming about 183 billion kilowatt-hours of electricity in the United States per year, according to a 2001 analysis by the Energy Information Administration. Generating that much electricity creates about 119 million tons of carbon dioxide, roughly the amount of CO2 spewed by 20 million cars each year. Remarkably, the percentage of homes with central air more than doubled, from 27 to 55 percent, between 1980 and 2001.

When deciding on how to air-condition your abode, you have two basic choices: single-room units or a centralized system. The single-room air conditioners are heavy, steel boxes designed to wedge into a window and, in high-rises, dangle precariously over passing pedestrians. (In case you were wondering: Yes, there are cases when they have fallen on people.) Inside, there’s a condenser, an evaporator, a thermostat and a couple of fans.

Central air conditioners have a different structure altogether. The condenser typically sits on the roof or in the back yard. A set of pipes runs coolant from the condenser into an air handler, often located near the home’s furnace. The cooled air is blown through a duct system and into individual rooms.

A central air system’s energy efficiency is measured two ways. The more basic rating is the energy efficiency ratio, or EER, which describes the unit’s energy consumption while cooling a prescribed volume of 95-degree air over the course of an hour in a specialized laboratory. The seasonal energy efficiency ratio, or SEER, is a bit more complicated and considers the average energy use at various temperatures and humidity levels.

(MIichael Sloan for The Washington Post)

Window units are rated only by EER, and lose out to central A/C by this metric. An Energy Star-certified central unit must have a minimum EER of 12, while window units only need only to achieve between 9.4 and 10.7, depending on size, to be certified.

One reason window air conditioners have lower ratings is that it’s impossible to fit much advanced hardware into that little box. While modern central air conditioners can work at a range of speeds, for example, the condensers in most window units have only two: on and off.

This can make a significant difference in energy use. Air conditioners not only lower air temperature; they also remove moisture. Just how much moisture is removed depends on how much air passes through the air conditioner. A machine that works only at full speed (or not at all) can drop a room’s temperature quickly by cooling just a small volume of air to an extremely cold state. In contrast, an air conditioner with variable speed settings can have the same effect by processing a larger volume of air but cooling each unit of it by a smaller amount. The larger volume results in greater dehumidification.

While this seems like a technical point, it’s quite significant, because it means users can set central air conditioners to a comparatively high temperature and feel comfortable, because the air is less humid.

But the window units have their advantages, too. Central air conditioners suffer from “duct losses”: As cooled air passes through the ducts, it warms up and often leaks through the fittings. These losses can cut energy efficiency by up to 30 percent.

Window units that buzz and shake violently every time the condenser switches on are basically begging for attention: Very few energy-conscious consumers walk out the door without remembering to shut off their window units. The quiet and unobtrusive functioning of central air conditioners, on the other hand, can lead to overuse, or overcooling when no one is home. Industry observers note that people tend to set their central air conditioner to a comfortable temperature and never change it.

Window units also offer room-by-room control. For singletons or couples who occupy only one bedroom at night, cooling a single room consumes a small fraction of the energy that a central air conditioner would.

Central air units are getting smarter, though, and accordingly more efficient. Programmable thermostats can make up for forgetful central A/C users by shutting the unit off when no one is home. Some systems divide homes into separately controlled zones, so you don’t have to cool the kitchen at 3 a.m. just to keep your bedroom a sleepable temperature. Newer central air conditioners can provide an air handler in each room, skipping the ducts entirely, combining precise control with improved efficiency over older central systems and less noise than a window unit.

Because of all the variables, it’s impossible to pronounce a winner in this particular competition. As is often the case, the choice is going to depend on each consumer’s behavior. Generally speaking, the newfangled ductless system is going to be the most efficient but priciest option upfront. (As a crude approximation, a ductless system costs about 30 percent more than a traditional central air conditioner for a house of the same size.) If you’re choosing between an old-school central air system and window units, the latter could be better if you’re childless, live in a large home in a dry environment or tend to be forgetful about adjusting your thermostat.

The Lantern thanks Jennifer Amann of the American Council for an Energy-Efficient Economy for help on this column. The Green Lantern is produced by the Web magazine Slate and can be read at