Energy efficiency of an A/C system directly affects its operating costs, both in terms of the owner’s electrical bills and society’s energy consumption. In a developed city, much of the energy consumption goes to A/C systems, and inefficient A/C systems put a tremendous amount of load on the city’s resources.

The simplest way to measure efficiency is to use EER, which is the rated capacity output divided by the rated power input. This simple formula measures the amount of cooling/heating capacity we get out of the A/C for every unit of power we put in, and can be quickly calculated based on the A/C’s specifications. Good A/Cs would have EERs above 2.5. To extend EER further and calculated the monetary costs, we can use the following:

Electricity cost = (Capacity Output / EER) x time in use x per unit cost of electricity

The compressor is the A/C component that most affects system efficiency. It can be considered the heart of the A/C system and provides the work input necessary to make an A/C work. Most compressors today use one of two technologies: the Scroll and the Rotary. To find out how the two compare in energy savings, let us use EER as a simple measure of efficiency. Typical 3HP Copeland scroll compressors have an EER of 3.3 while typical 3HP rotary compressors have an EER of 2.99. Assuming we need a capacity output of 7.1 kW, use the A/C 10 hours a day, and spend 15 cent per kWh of electricity. The electricity per day for each technology is then:

Scroll’s cost = 7.1/3.3 x 10 x 0.15 = $3.22
Rotary’s cost = 7.1/2.99 x10x0.15 = $3.56

We see clearly that the scroll gives us a cost reduction of 10% in this example, a significant savings.

Reliability is a factor that is hard to measure, yet extremely important in selecting an A/C. Unlike household appliances, an A/C is not simply a product in and of itself; it requires maintenance and servicing to be complete package, and therefore depends heavily on reliability. An unreliable A/C would required a lot more maintenance and repairs, and hence incur a lot more cost in after sales service.

Most common causes of failure are related to the compress and, hence, a reliable compressor is essential to having a reliable A/C. Generally speaking, the Scroll is more reliable than the rotary system. The Copeland Scroll is a very reliable system as there are 70 million scroll compressors installed in the field for the past 20 years worldwide. This puts the Copeland Scroll’s reliability above other scrolls.

How do we measure A/C reliability in general? Unfortunately, no clear and hard figures can measure reliability, so research into any brand and model one considers is important. Warranty is naturally the first step, but oftentimes warranties are similar across manufacturers. More telling is the reputation of the A/C brand, the A/C model, and the brand of important parts (such as compressor). On top of these, one must also remember that reliability comes at a cost. Any A/C that is significantly less expensive than others of similar specifications is suspect.

The ultimate purpose of most A/Cs is to bring comfort to the users, but comfort can be defined in many ways. Temperature is the first step, but beyond that we have to consider things such as temperature fluctuations, fan direction and speed, humidity, etc. For instance, an average fixed speed A/C can keep room temperature fluctuations to within a range of 4 °C, not a comfortable range for most people. Central A/C VRF system, however, can keep to within a range of 1 °C, which is not noticeable to most.

Another important comfort factor is the speed at which desired temperature is reached. Naturally, when we enter a hot room and turn on the A/C we would like the room to cool down to our desired temperature rapidly. Therefore, we must also consider the rate at which any system cools or heats a room to the desired temperature.

Comfort is especially important in residential A/Cs, and we see that it is an increasingly important consideration for middle/high-end residences. Generally, Central A/C VRF system can are more comfortable than fixed-speed systems, especially when the design allows for various ways of delivering air to the room.

Initial cost is usually the first consideration of many when they choose an A/C system but this is a limited view. In actual fact, costs come in many forms: operating costs, maintenance costs, etc. As discussed earlier, efficiency and reliability also affects the ultimate cost greatly. Therefore, to fully understand the cost of an A/C, all the above considerations must be taken into account.

We have already examined the energy savings for scroll vs rotary systems. Let us consider the fixed capacity system vs variable capacity system. To fully understand the efficiency difference between the two, we need to understand that a fixed capacity system maintains temperature by the compressor switching on and off. This turning on/off process takes up a lot of power and leads to substantial costs. Moreover, as the fixed capacity system cannot adjust capacity to match requirements, a lot of power is wasted when needed capacity is below the actual capacity output of the fixed speed system. Variable capacity system using Copeland Digital Scroll can save energy as much as 40% vs fixed capacity system.