InspectAPedia ^®

This article explains air conditioning SEER energy efficiency ratings along with related terms like Watts and hourly operating cost, in easy to understand language. For a history of the US Government's use of energy efficiency ratings for air conditioners, also see SEER RATING HISTORY. This website answers most questions about air conditioning systems. We continue to add to and update this text as new details are provided. Contact us to suggest text changes and additions and, if you wish, to receive online listing and credit for that contribution.

© Copyright 2009 Daniel Friedman, All Rights Reserved. Information Accuracy & Bias Pledge is at below-left. Use links at the left of each page to navigate this document or to view other topics at this website. Green links show where you are in our document or website.

SEER RATINGS EXPLAINED - determining Seasonal Energy Efficiency of Air Conditioning Systems

SEER stands for "Seasonal Energy Efficiency Ratio. This is a measure of the energy efficiency of the air conditioning system. SEER ratings permit consumers to compare operating costs of various cooling systems and products.

SEER = [Total Cooling Output Over the Cooling Season] / [Total Electrical Energy Input Over the Cooling Season]

Higher air conditioning SEER rating means more efficient, or in other words lower energy cost to cool the building. Older air conditioning systems are likely to have a lower SEER (perhaps 5 or 6) than a newer more efficient system (perhaps SEER=10). But beyond comparing SEER ratings, a look at the building insulation, air leakage, and the layout, insulation, and adequacy of the air conditioning duct system are likely to have a very large, usually determining effect, on the operating cost of air conditioning systems in buildings.

How Much Energy Does an Air Conditioning System Use?

How to Calculate Energy Usage using an Air Conditioner's SEER rating

A concise way to translate SEER number directly into energy cost is SEER 10 = 10 BTUs/WattHour. In other words, an air conditioner that has a SEER rating of 10 will provide 10 BTUs of cooling per WattHour (Wh) of operation. So if our air conditioner has a SEER of 9, it is less efficient than an A/C unit with a SEER of 10 because our SEER 9 air conditioner produces 9 BTUs of cooling for the same Wh of operation. That is, we've kept the energy consumption (one Wh) the same, but we got less cooling output.

Let's define Watts and BTUs so we can better understand these air conditioner figures of SEER efficiency, BTUs, Watts, and air conditioning operating cost calculations.

What is the SEER for Older Air Conditioners?

Older air conditioning systems are likely to have a lower SEER (perhaps 5 or 6) than a newer more efficient system (perhaps SEER=10). But beyond comparing SEER ratings, a look at the building insulation, air leakage, and the layout, insulation, and adequacy of the air conditioning duct system are likely to have a very large, usually determining effect, on the operating cost of air conditioning systems in buildings.

Air Condition, Refrigeration, & Heating System Standard Definitions

What is a Watt Hour or Wh?

What's a WattHour? Watt hours (Wh), sometimes written W.h, can measure either electrical energy produced, say by a power station, or Watts can measure the amount of electrical energy consumed (say at a light bulb or an air conditioner in our home). For air conditioners, the A/C units' total Wh is the energy used in running the air conditioning system for an hour.

If you turn on a 100-watt light bulb for an hour, you've used 100 Wh of energy. Or if you had a one-watt bulb and lit it for an hour, it'd use 1 Wh of energy. Thank James Watt (1736-1819), credited with developing a useable steam engine, for WATT which was named for him in 1882.

How do we calculate watts, volts, and amperage for an electrical device like an air conditioner?

Watts (W) is a measure of electrical power and is expressed by any of these formulas:

W = V x I

W = I² x R

W = V² / R

W = Watts, V = Volts, I = Current or Amperage or Amps and R = Resistance measured in Ohms

Example: if we have a 50 watt light bulb running on a 120V circuit we can solve for the missing number, I or "Amps"

50 = 120 x I

50 / 120 = I

0.416 = I

Our 50 watt light bulb is drawing .4 amps of current.

Reader Michael V. points out that in the above watts, volts, amps calculations, these simplified formulas are for DC voltage. In AC,  V*I=VA  not watts.   Watts  is  W=V*I*PF  where PF =powerfactor. Also see DEFINITIONS of ELECTRICAL TERMS and see AMPS & VOLTS DETERMINATION "How to estimate the electrical service ampacity and voltage entering a building".

We're simplifying in this example; in real life there is some voltage drop across an electrical circuit (such as a lighting circuit), and the resistance of the circuit in ohms will include not only the resistance of the electrical device (say the light bulb) but also the wire in the circuit. That's why using certain multimeters that promise to detect an unsafe aluminum wire circuit connection can be unreliable. The tester cannot tell the difference between a slight increase in resistance at an electrical connection and the slight resistance caused by the length of copper wire in the circuit.

Watts is an instantaneous measurement, not related to time. To factor in time, as the electrical utility wants to do in sending us an electrical bill, the electric company's meter calculates the number. of watt hours (actually kilowatt hours) of electricity we use. If we run our 50 watt bulb for one hour, we've used 50 watt-hours. That's all the electric utility cares about.

Lots of electrical appliances include a label providing the appliance's wattage, and in the case of heating and air conditioning equipment, lots of other details are provided too. See A/C DATA TAGS for details.

What is a BTU or British Thermal Unit? What is a Joule?

Definition of BTU

A BTU is a measure of heat energy. One BTU is the amount of heat energy we need to raise the temperature of one pound of water by one degree Fahrenheit. One BTU also is defined as 252 heat calories (this is not the same as food calories). When talking about air conditioners or heaters, we talk about the A/C unit's BTUh capacity - the number of BTUs of cooling (lowering rather than raising temperature) it can produce in an hour of running.

Here is a table of BTUs translated into other measurements:

Other definitions related to BTUs and heating or air conditioning:

The latent heat of vaporization is defined as the number of BTUs to raise one pound of liquid to a pound of vapor (to a varying degree per BTU depending on the type of vapor - this is "superheat") Sketch courtesy of Carson Dunlop.

The latent heat of condensation is defined as the number of BTUs necessary to change a state back from a vapor to a liquid

The latent heat of solidification is defined as the amount of energy (or number of BTUs) needed to change a liquid to a solid (such as water to ice) while the temperature remains unchanged (at sea level, 32 degF).

Sensible heat is defined as the amount of heat that we can sense or feel or measure.

Latent heat is defined as the amount of heat absorbed by a substance with no change in a temperature - such as when a substance changes state (from water to steam, for example)

Specific heat is defined as the amount of heat required to raise the temperature of a given substance by one unit of temperature (in our examples by one degF.) Specific heat is also defined as the amount of heat (in calories) to increase the temperature of one gram of a substance by one deg C (Celsius).

The specific heat of water is 1
The specific heat of ice is is 5

In which direction does heat flow: heat energy always flows from the warmer substance to the cooler substance, down to -460 degF where all molecular movement stops.

A neat fact is that the heat flows more rapidly (efficiently) between two substances when there is a greater temperature difference between them. That's why the thermal conductivity of finned copper tubing heating baseboard is exponentially greater at higher degrees of heating water temperature, and that's why we like to run our heating boiler at a higher rather than a lower upper limit temperature.

Definition of Joule

Note: Outside of the U.S. and some other places, BTUs is being replaced with the SI unit of energy, the Joule. (J). The English have beaten out the Scots by James Prescott Joule who defined this value. since there are 3600 seconds in an hour) the following formulas equating Watts, Joules, and Newton meters can be written:

1 Watt second (Ws) = 1 joule (J) = 1 newton meter 1 Watt hour (Wh) = 3600 Joules

1 kilowatt hour = 3.6 x 10⁶ Joules, since there are 1000 watts in a kilowatt.

We can think of an air conditioner's "efficiency" as expressed either in the total operating cost for a season of use, or you may prefer to just express the air conditioner's efficiency as its operating cost to run the system for one hour.

The equation shown at page top is designed to reduce all of the parameters describing air conditioning efficiency to a single efficiency number, SEER. SEER numbers are useful when we're comparing one air conditioner with another. But suppose we want to know the actual air conditioning cost per season, or air conditioning cost per operating hour to operate our air conditioner?

To translate our air conditioners SEER rating into actual air conditioning operating costs we need to know:

How do We Translate BTUs to Tons of Air Conditioning or Cooling Capacity?

One ton of air conditioning capacity produces the same cooling ability as melting one ton of ice in 24 hours. Sketch courtesy of Carson Dunlop

288,000 BTUs / 24 hours = 1 Ton of cooling

12,000 BTUs / hour = a 1-ton air conditioning system

Or if we know the total number of BTUs at which an air conditioning system is rated, since this number is usually given in BTUH or BTUs / hour, we just divide that number by 12,000 to get the number of tons of cooling capacity.

A 36,000 BTUh air conditioner is providing 36,000 / 12,000 or 3 Tons of cooling capability per hour.

If we know the number of tons of cooling capacity that an air conditioning system is rated for, we just multiply the number of air conditioning capacity in Tons by 12,000 to get the number of BTUs of cooling capacity of the system.

A 3-ton air conditioner is providing 3 x 12,0000 or 36,000 BTUs of cooling capability per hour.

To assist in choosing the right sized air conditioner, we provide a typical air conditioner chart at AIR CONDITIONER BTU CHART.

Don't buy an air conditioner that is too big: if you install a system that is too powerful (too many tons of cooling capacity) the building will be less comfortable than if you install a properly-sized air conditioner. Too many tons of air conditioning mean the system will shut off on short cycles and won't run long enough to reduce the indoor humidity to a comfortable level.

How Much Electricity Does An Air Conditioner Use Per Hour?

How much electricity our air conditioner uses per hour is easy to calculate. Let's assume that the data tag on our air conditioner says that the unit is a 5000 BTUh device with a SEER rating of 10. This means our A/C unit will produce 5000 BTUs of cooling in an hour of running. Since SEER=10 means that 10 BTUs used per Wh, then

5000 BTUh / 10 SEER = 500 Watts per hour that our A/C unit will use.

How Much Electricity Does An Air Conditioner Use in one Cooling Season?

A common example we use (because the math is easy) is to assume we have 125 days of cooling season during which we run the air conditioner for eight hours per day.

8 x 125 = 1000 hours of cooling operation over a season
500 Wh (watts used per hour) x 1000 (hours per season) = 500,000 Wh per season

So we are using 500,000 Watt Hours of energy (electricity) per cooling season. We divide this by 1000 to convert to Kilowatts since that's how our electrical bill will express our electricity usage.

500,000 Wh / 1000 = 500 kWh or kilowatt hours per season of use
That's how much electricity we're using over the cooling season.

What is the Definition of High Side and Low Side in Air Condition & Refrigeration Systems?

Definition of Low Side in an Air Conditioning System refers to the components on the low-temperature and low-pressure side of the compressor unit. In an air conditioner, the low side includes the suction or intake side of the compressor unit, suction piping connected to the evaporator coil, the evaporator or cooling coil, and the output-end of the metering device or TEV.

Definition of High Side in an Air Conditioning System refers to the components on the high-temperature (above ambient air temperature) and high pressure side of the compressor unit. In an air conditioner in cooling mode these include the output or high pressure side of the compressor unit, the high pressure gas refrigerant line connected to the condensing coil, the condensing coil itself, and the inlet side of the metering device located near the evaporator coil.

These parts are named and illustrated at AIR CONDITIONING SYSTEMS and at COMPRESSOR PRESSURE READINGS we discuss air conditioner system high side and low side further.

In the next section of this article we determine the actual dollar cost of running an air conditioner either by the hour of by the season of use. It's easy to get from that data to actual air conditioning operating costs in dollars.

Share this Article      

...

Technical Reviewers & References

Search InspectAPedia

• Daniel Friedman - principal author/editor of the InspectAPedia^® Website
• InspectAPedia Bookstore lists recommended books, organized by topic & available for purchase. Most of our articles also include a list of recommended books for the specific article topic as well as other references, and information sources.
• Critique, contributions wanted: Contact Us to suggest corrections or additions to articles at this website, and if you wish, to receive online listing and credit as a contributor. Particular thanks are due to the many experts and also consumers who read and critique technical articles at InspectAPedia.com.
• Additional technical contributors & reference sources for this article are listed below.

Use links just below or at the left of each page to navigate this document or to view other topics at this website. Green links show where you are in our document or website.

AIR CONDITIONING & HEAT PUMP SYSTEMS
AGE of AIR CONDITIONERS & HEAT PUMPS
AIR CONDITIONER BTU CHART
AIR CONDITIONER COMPONENT PARTS
AIR CONDITIONER TYPES, ENERGY SOURCES
AIR CONDITIONER NOT WORKING
AIR FILTERS for HVAC SYSTEMS
AIR HANDLER UNIT
BACKUP HEAT for HEAT PUMPS
BLOWER DOORS & AIR INFILTRATION
BOOKSTORE - Air Conditioning "How To" Books
CLEANING & Legionella BACTERIA
COMPRESSOR CONDENSER
CONDENSATE HANDLING
CONTROLS & SWITCHES
COOL OFF HEAT Thermostat Switch
COOLING CAPACITY, RATED
COOLING COIL or EVAPORATOR COIL
DATA TAGS on AIR CONDITIONERS
COMBUSTION GASES & PARTICLE HAZARDS
COMBUSTION PRODUCTS & IAQ
DEFINITION of Heating & Cooling Terms
DEW POINT CALCULATION for WALLS
DEW POINT TABLE - CONDENSATION POINT GUIDE
DUCT SYSTEMS
DUCTS - Asbestos
DUCT SYSTEM DEFECTS
DUCT INSULATION, Asbestos Paper
DUCT INSULATION for SOUNDPROOFING
DUCTS, Asbestos Transite Pipe
DUST FROM HVAC?
ENERGY SAVINGS in BUILDINGS
EVAPORATIVE COOLING SYSTEMS
FAN AUTO ON Thermostat Switch
HEAT LOSS (or GAIN) in BUILDINGS
INDOOR AIR QUALITY & HOUSE TIGHTNESS
INDOOR AIR QUALITY IMPROVEMENT GUIDE
INSPECTION LIMITATIONS
LOST COOLING CAPACITY
MOTOR OVERLOAD RESET SWITCH
OPERATING COST
OPERATING DEFECTS
OPERATING TEMPERATURES
REPAIR GUIDE for AIR CONDITIONERS
REPAIR & DIAGNOSTIC FAQs for A/C
REFRIGERANTS
  REFRIGERANT LEAK DETECTION
  REFRIGERANT LEAK REPAIR
SEER RATINGS & OTHER DEFINITIONS
  SEER RATING HISTORY
SYSTEM OPERATION
SWAMP COOLERS
THERMOSTATS
THERMOSTATIC EXPANSION VALVES
CRITICAL DEFECTS
Air Conditioning "How To" Books

• Thanks to Mark Cramer, Tampa Florida, for assistance in technical review of the "Critical Defects" section and for the photograph of the deteriorating gray Owens Corning flex duct in a hot attic. Mr. Cramer is a Florida home inspector and home inspection educator.
• Thanks to Alan Carson and Bob Dunlop, Carson Dunlop, Associates, Toronto, for permission to use illustrations from their publication, The Illustrated Home which illustrates construction details and building components. Carson Dunlop provides home inspection education, publications, report writing materials, and home inspection services. Alan Carson is a past president of ASHI, the American Society of Home Inspectors.
• Thanks to Scott at SJM Inspect for suggesting this EPA document and for technical editing remarks regarding our air conditioning website, SJM Inspection Service LLC, serves the entire state of CT, sjminspect@optonline.net 203-543-0447 or 203-877-4774 5/16/07
• Thanks to reader Michael V. for commenting on watt, volt, amp calculations, August 2009.

Books & Articles on Building & Environmental Inspection, Testing, Diagnosis, & Repair

• Our recommended books about building design, inspection, and repair, and about indoor environment testing, diagnosis, and cleanup are at the InspectAPedia Bookstore.
• Complete List of Air Conditioning & Heat Pump Design, Inspection, Repair Books at the InspectAPedia Bookstore.
• Modern Refrigeration and Air Conditioning, A. D. Althouse, C.H. Turnquist, A. Bracciano, Goodheart-Willcox Co., 1982
• Principles of Refrigeration, R. Warren Marsh, C. Thomas Olivo, Delmar Publishers, 1979
• "Air Conditioning & Refrigeration I & II", BOCES Education, Warren Hilliard (instructor), Poughkeepsie, New York, May - July 1982, [classroom notes from air conditioning and refrigeration maintenance and repair course attended by the website author]
• Refrigeration and Air Conditioning Technology, 5th Ed., William C. Whitman, William M. Johnson, John Tomczyk, Cengage Learning, 2005, ISBN 1401837654, 9781401837655 1324 pages
• Carson Dunlop, Associates, Toronto, have provided us with (and we recommend) Carson Dunlop Weldon & Associates' Technical Reference Guide to manufacturer's model and serial number information for heating and cooling equipment ($69.00 U.S.).
• ...

More Information on Building Diagnostic Inspections and Repairs

• Air Conditioning SEER - New DOE Air Conditioner and Heat Pump Efficiency Standard offers more details about SEER and the phase out of older refrigerants (like R-22)
• Air Conditioning Inspection, Diagnosis, Repair, Efficiency all the basics for home owners, inspectors, new repairmen
• Air Conditioning SEER - New DOE Air Conditioner and Heat Pump Efficiency Standard
• Asbestos HVAC Ducts and Flues field identification photos and guide
• Fiberglass: Indoor Air Quality Investigations: Fiberglass in Indoor Air, HVAC ducts, and Building Insulation

InspectAPedia® Home & Site Map - Building & Environmental Inspection, Testing, Diagnosis, Repair, & Problem Prevention Advice: In-depth research & advice on diagnosing, testing, correcting, & preventing building defects & indoor environmental hazards. Unbiased information, no conflicts of interest.

The Mold Information Center: What to Do About Mold in Buildings, When and How to Inspect for Mold, Clean Up Mold, or Avoid Mold Problems

Use this simple, economical mold test kit by following our instructions on how to collect and mail mold samples to our lab

Environmental Inspection, Testing, & Diagnosis On-Site IAQ, Gas, Air Testing, Mold Investigation, Sick Building Diagnosis, Lab Services, & Remediation Plan Preparation - indoor air quality testing, problem source determination, supporting lab work, written remediation plan addressing removal of environmental and other hazards and prevention of their recurrence.

Building Inspection, Problem Diagnosis, Forensic Investigation & Testing, Repair Consulting

Contact Daniel Friedman for website content suggestions or for fee-paid consulting