A two-stage air conditioning unit offers more energy efficiency while making it easier to stay within a narrow range of temperatures. Having the second piston also lets the two-stage system move air along more quickly, as the second piston only operates when temperatures become quite hot.
Single-stage air conditioning systems are less expensive. However, it costs a lot more to operate them through the entire life of the system. The fact that the system is either on or off all the time keeps temperatures going up and down. When the unit powers on each time, it goes into full power, costing more at the power meter, particularly during summer's hottest days.
A two-stage system, in contrast, operates at two different speeds. The low setting is sufficient to keep the house cool about 80 percent of the time. While the two-stage systems do stay turned on longer, they also provide a temperature range that is more even, which means that they spend significantly less time at full power than a single-stage system. Because of this increased efficiency, they actually use less electricity, even though the motor is on longer. Over time, the electricity savings often pay for the difference associated with purchasing the more expensive two-stage system.