Now let’s take a look at each of them in turn to provide a better understanding of how they can be used to individually measure a system’s performance.

The Energy Efficiency Ratio (EER) is the most commonly used performance measure. It is defined as the ratio of cooling capacity in Btu/h to total electrical energy input in watts under average outdoor conditions. EERs are mostly used for air conditioning and refrigeration equipment.

The Seasonal Energy Efficiency Ratio (SEER) is another common method that measures a unit’s ability to provide heating or cooling amount of energy over an entire cooling season, expressed in units of Btu/Wh (Btu per watt-hour). The higher the SEER rating, the more efficient the system. In order to achieve high-efficiency levels when buying a new system, you’ll need systems with SEER ratings of at least 14.

In general terms, here’s how SEER works: Let’s say you have two identical units installed side-by-side in the same house. One unit has a 10 SEER rating and the other has a 13 SEER rating. If both units are operating 24 hours per day during the cooling season, the unit with a 13 SEER will be more efficient–it will use less electricity to do the same amount of work as the system with a 10 SEER capacity.

Coefficient of Performance (COP) measures the efficiency of a chiller by taking into account everything that goes on inside the machine. The COP is equal to the heat removed from the conditioned space multiplied by the work required per pound of refrigeration (the “duty”).

To illustrate, let’s say you have two chillers with 10 tons of capacity each and they’re both capable of removing 10 ton-hours of heat per day. If one chiller requires 30 kWh/ton to remove the heat while the other requires 20 kWh/conventional ton, then at 100% duty cycle (running continuously) they will consume 30 kWh for every 10 ton-hour removed from the conditioned space with no consideration given to electrical consumption.

Since this comparison is based on quantity rather than efficiency it doesn’t provide any insight into which chiller is more efficient.

kW per ton of Cooling capacity is a measure of efficiency that can be used on both centrifugal and screw chillers. In this case, the kW/ton ratio indicates the amount of conditioned capacity being removed from a space per unit of energy consumed by the chiller. The higher the number, the more efficient the chiller is at removing heat. One must keep in mind that it functions as if all power were drawn from one source so kW/ton may include demand charges where applicable.

Heating Seasonal Performance Factor (HSPF) is used to rate the energy efficiency of a furnace. The HSPF is a unitless number that describes how well an appliance will provide heat during the heating season. The higher the number, the more efficient the appliance will be at providing heat. The standard for measuring HSPFs has been set by the Residential Heating System Specification Program (RHSS) and is used to measure furnaces with a capacity of 65,000 Btuh or less.

HSPF = Total Heating Load / Total Energy Input

It’s no surprise that there are two different standardized sets of criteria for HVAC performance. The Air Conditioning Heating & Refrigeration Institute (AHRI) has developed testing procedures to measure packaged units under 65,000 Btu while the International Organization for Standardization (ISO) creates standards on how to measure larger packaged and central systems.

In addition, they also outline other factors which can affect whether an air conditioner operates efficiently such as its size, demand level during peak times versus off-peak hours, etc.