What do the HVAC Energy Ratings Mean?

Published on: June 6, 2013

Every air conditioning and heating system carries one or more energy rating. While these numbers make it easier to find the right HVAC system for your home or office, they can also be confusing when you first see them. The alphabet soup of acronyms and terms from physics classes do provide a wealth of information once you understand what they mean.

Energy ratings are not just abstract terms. They have a significant impact on the size of your utility bills and the comfort your air conditioner provides. According to the Environmental Protection Agency’s Energy Star site, each point of improved Seasonal Energy Efficiency Ratio rating, or SEER, represents about a 5 percent decrease in your utility bill. You may also qualify for a tax credit when you replace an inefficient unit with a more efficient one.

British Thermal Units

To a physicist, both heaters and air conditioners perform the same task of thermal transfer; they just do it in opposite directions. The amount of energy transferred by the AC, heater or other appliances is often measured in British thermal units, or BTU. One BTU is the amount of energy required to bring the temperature of a pound of water up by a single degree Fahrenheit. An individual BTU is tiny; most air conditioners put out thousands of BTU’s per hour.

Another term you’ll see with air conditioners is a measurement in tons. While this is not a direct measure of efficiency, it can be useful for finding BTU output if the unit isn’t labeled. Tons originally came from the amount of ice needed to cool a space. The measurement is analogous to using horsepower to describe an engine’s power; no car was ever pulled by 250 horses, but the measurement was handy for visualizing an abstract concept. One ton is equivalent to 12,000 BTU.

Energy Efficiency Ratio

Dividing the hourly BTU output of a system by the wattage it consumes per hour gives you the EER. This measurement assesses a system’s peak performance and is more often used for window units than for central air conditioners. Higher EER ratings signify greater efficiency. For example, if your air conditioner put out 45,000 BTU and used 450 watts per hour, it would have an EER of 10.

For hot climates in which an air conditioner will run for long periods of time, the EER rating is an effective gauge of efficiency. In temperate climates, another related measurement presents a more accurate picture.

Seasonal Energy Efficiency Ratio

Like the EER, the SEER divides BTU output by energy input. The difference is that the SEER adds a time component and measures overall efficiency. If EER is a single photograph, then SEER is a feature-length movie that tells a more detailed story about energy use. Most heating systems and central air conditioning units use this rating instead of the older EER. Over the course of a year, the SEER rating may fluctuate by as much as 25 percent for a given EER rating because systems do not always operate at peak efficiency.

SEER ratings have become the industry standard for air conditioners and range from a low of 13 to a high of 26 or so. Older systems may have a lower SEER rating, but the EPA requires that all new central AC units have a minimum rating of 13. By 2015, that minimum standard will rise to 14 in hot southern states.

Heating Seasonal Performance Factor

Heat pumps often measure efficiency with the HSPF. It is functionally identical to SEER ratings and is also a ratio of BTU to watts per hour consumed, but because manufacturers measure it at a different set of controlled temperatures, the figures are slightly different than SEER ratings. An HSPF of 8 or higher is considered efficient.

EnergyStar Label Requirements

The EPA has developed the EnergyStar program to help buyers choose efficient appliances and HVAC systems. Window units must be a minimum of 10 percent more efficient than current government standards for room units. The requirements for whole-home systems are more detailed. Single-package equipment trades a small amount of efficiency for economy and ease of installation, but split systems offer slightly better efficiency in the long run.

Heat pump, split system            12    14.5    8.2
Heat pump, package                  11    14       8
Air conditioner, split system       12    14.5   n/a
Air conditioner, package             11    14      n/a

Actual Efficiency vs. Theoretical Efficiency

Efficiency ratings are predicated on set temperatures and don’t always reflect the actual efficiency your system will deliver. If your home has large windows that let in plenty of sunlight, your air conditioner may not function as efficiently as its label suggests; your heater, on the other hand, could become slightly more efficient than its stated HSPF with help from the sun. Your annual savings will also depend on how much you currently pay for power. If your cost per kilowatt is low, then your new AC system may take longer to pay for itself.

Maintenance is key to reaping the benefits of a more efficient heating and cooling system. Neglect can rob your AC of as much as 5 percent of its efficiency per year. Changing filters, cleaning coils and insulating ductwork properly will make an older system more efficient and keep a new AC operating at close to its theoretical maximum. After investing in an EnergyStar system, schedule regular maintenance to keep it in good condition.

If you have any questions about efficiency ratings, system sizes, maintenance recommendations or any other aspect of residential or commercial heating and cooling, the certified technicians at AC Southeast® can answer them. When you call for an installation estimate, your technician can also help you calculate your expected savings on systems with different EER/SEER ratings.