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A new air conditioning unit is a big purchase, so it’s important to do careful research before choosing one. You may be unsure of whether a single stage or variable speed AC is better for your home. To pick the best option, you should understand the difference between the two as well as the pros and cons of both choices.

what are stages in air conditioning?

1. Single Stage A single stage air conditioners power is either ON or OFF. When a single-stage air conditioner turns on, it can only run at 100% capacity. Therefore,they are called “single-speed”: they run at one speed only which is FULL ON!These capacity limitations result in significant wasted energy and lower levels of comfort since the system often “short cycles” by satisfying the thermostat call too quickly.

2. Two-Stage

When a two-stage AC turns on, it can run at 1 of 2 speeds:

  • HIGH: the AC runs at 100% capacity
  • LOW: the AC runs at 60–70% capacity

To address the deficiencies in single stage units, manufacturers developed two-stage air conditioners and heat pumps that incorporate a single scroll compressor modified with a solenoid control and a plate with two different sized holes drilled into it. The solenoid shifts the position of the plate based on the mode the thermostat calls for. The bigger hole is used for higher demand, and conversely, the lower hole is used for lesser demand. During mild days, the compressor operates at reduced capacity and uses less energy as the building cools more gradually. When the temperature rises, the unit runs at full capacity to meet the demand.

3. Variable Speed

Variable-speed compressors are the latest innovation in HVAC energy efficiency. These units regulate the output of the compressor though frequency modulation guided by a Variable Frequency Drive (VFD). The compressor can modulate up or down to match the cooling or heating requirements of the building. Modern variable-speed compressors provide 70-700 different speeds of operation. The compressor has the capacity to change speeds in increments as small as 1/10th of one percent, so the actual indoor temperature varies by less than one-half degree of the thermostat setting.

The term “variable-speed” applies to two different components in an air conditioning or heat pump system: the blower motor and the compressor. A variable-speed blower can be matched with a single-speed compressor, but a variable-speed compressor must always be paired with an adjustable-speed blower.

the difference between single stage and variable speed

The difference between a single stage and a variable speed unit is in the way the compressor functions. The compressor is the core component of the AC unit. It circulates the refrigerant through the rest of the system, which is how the AC removes heat from the inside of your home.

A single stage compressor only operates at one speed, but a variable speed compressor can run at different capacities. A variable speed model usually can adjust its speed to anywhere between 25 and 100 percent capacity depending on how much energy is needed to cool the home. A single stage system, on the other hand, either operates at 100 percent or at 0 percent.

the pros and cons of single stage compressors

A single stage compressor is the least expensive system to buy and install. If you are on a tight budget for your new AC unit, a single stage system may be appealing in terms of upfront cost.

The biggest downfall to only running at 100% capacity is that these units turn on, cool the home very quickly and then shut off. This ON then OFF cycle repeats over and over throughout the day. This process is called short-cycling.

And, because they short cycle so much, single-stage ACs:

  • Cannot dehumidify very well
  • Have shortened life spans
  • Consume a lot more energy (i.e., high monthly energy bills)
  • Produce hot/cold spots
  • Cannot provide precise temperatures (they often over/undershoot the set temperature by up to 4–6°)

the pros and cons of Two stage compressors

In Arizona we have temperature swings throughout the year. During the winter we have mild climate, but spring and fall we can still see temperatures in the 80’s and 90’s while the summers of course can get as hot as 115 degrees. What is nice about two stage is that if can be adjusted to better match the demand for what is happening outside and keep your system running at lower settings.

Because they run at a lower setting, they also have longer run times—which may sound bad but it is not!

In fact, because two-stage ACs have longer run times, they:

  • Provide better dehumidification
  • Live longer
  • Need less repairs
  • Consume less energy
  • Provide more precise temperatures (can cool your home +/- 2–4° from set temperature)

the pros and cons of variable speed compressors

Variable speed compressors are much more energy-efficient than single stage systems. If your home only needs to be cooled down by a couple of degrees, a variable speed unit will not waste energy running at full capacity. It also does not turn on and off as frequently as a single stage unit, which reduces energy usage.

Variable speed systems can cool your home more precisely, too. They can usually get within 1 degree or less of the set temperature, and they distribute the cool air evenly throughout your home, which can be a significant benefit if your home tends to have hot and cold spots.

Variable speed systems run at exceptionally low speeds and because they run at very low speeds the majority of the time, variable-speed AC’s have long, steady run times, which leads to:

  • Powerful dehumidification
  • Longer lifespans than single-stage/two-stage units
  • Less repairs/breakdowns than single-stage/two-stage units
  • Extraordinarily little energy consumption
  • Very precise temperatures (can cool your home +/- 0.5° from your set temperature)

The main downside to a variable speed compressor is the cost. Installation can be expensive in the beginning, and repairs can be more costly and complicated when the systems does need repairs so we recommend that you invest in a bi-annual maintenance plan with a local professional to stay on top of general maintenance and optimize your system.

However, variable speed systems can save you money over time on your utility bill and cool your home more effectively. If you plan to stay in your current home for a while and prefer keeping your home at a low temperature throughout summer, you will likely get your money’s worth from a variable speed AC.

So now that we know how these ACs work, let us compare how much they cost upfront.

How much do they cost: single vs two-stage vs variable speed?

To help you compare prices, let us look at 3 different full air conditioning systems and their approximate equipment and basic installation price: Note: The prices above reflect the approximate cost of the unit, labor, materials, tools, permits, etc.You can expect to add anywhere from $500 to $1,000 to the numbers above if your system requires more materials and labor for more difficult installations. Examples: cranes to lift units on top of roofs, difficult to work in spaces such as attics and crawl spaces, or ductwork fabrications so that your new system will fit the older pre-existing ductwork.

For more information on AC installation costs in Southern Arizona, feel free to call AIR4U at (888) 702-4748.

So, you can see that you would spend around $2000 more for a two-stage unit or $5,000 more for a variable-speed AC system (assuming installation costs remains the same for all air conditioners).

But remember—variable ACs offer lower monthly energy bills that could pay back that higher installation cost over the lifetime of the unit.  As much as $5,000 in the first five years.  Check out our energy saving calculator tool so see possible savings for various increases in efficiency/SEER ratings

Consult one of our Comfort Consultants to determine if/when a variable-speed AC would pay back its higher upfront cost given your cooling habits.

our recommendations

When choosing a new AC system, consider your budget, the size of your home, and the climate of your area. Weigh the pros and cons of both single stage, two stage, and variable speed units and consult with an HVAC professional to determine the best option.

Our advice on single-stage ACs? If you are on a super strict budget, this AC will “get the job done” but because a single-stage AC comes with high energy bills, we usually try to convince homeowners to spend a bit more upfront for at least for a two-stage AC.

Our advice on two-stage ACs? If you are on a budget, this is a great option because you get lower energy bills with a two-stage vs a single-stage. Plus, homeowners typically spend less on repairs over the lifetime of a two-stage AC (due to their ability to run on a lower, energy-efficient speed).

Our advice on variable-speed ACs? Get one if you can afford it! These advanced air conditioners offer huge savings on energy bills, need very few repairs (when used correctly and maintained regularly) and offer the highest comfort levels.


Whether you are ready for an estimate for your AC installation or have questions about single-stage, two-stage or variable-speed ACs, AIR4U can help.

Just give us a call at (888) 702-4748 or email us at