Shopping for an HVAC system? You'll encounter an alphabet soup of efficiency ratings: SEER, SEER2, EER, EER2, HSPF, HSPF2, AFUE, COP...

What do they all mean? Which ones matter? And why are there so many?

This guide breaks down every efficiency rating you'll encounter, explains how they're measured, and helps you understand which numbers to focus on for your specific needs.

Quick Reference Guide

SEER: Seasonal Energy Efficiency Ratio

What SEER Measures

SEER measures how efficiently an air conditioner or heat pump cools your home over an entire cooling season.

The formula:

SEER = Total Cooling Output (BTU) ÷ Total Energy Input (Wh)

Example: A system that provides 60,000 BTUs of cooling while consuming 5,000 watt-hours has a SEER rating of 12.

How SEER Is Tested

Manufacturers test systems under controlled conditions that simulate a typical cooling season:

• Temperature range: 65°F to 104°F outdoor temps
• Indoor conditions: Maintained at 80°F, 51% humidity
• Variable conditions: Tests at multiple outdoor temperatures, weighted by frequency
• Duration: Hundreds of hours of testing to simulate seasonal use

SEER vs SEER2 (What Changed in 2023)

In January 2023, the DOE introduced SEER2, an updated testing procedure that better reflects real-world conditions:

Key changes:

• Airflow rates: More realistic CFM (cubic feet per minute) measurements
• Static pressure: Accounts for typical ductwork resistance
• Fan power: Includes more accurate blower energy consumption

The result: SEER2 ratings are typically 4-5% lower than old SEER ratings for the same unit. A SEER 16 system might be rated SEER2 15.2 under the new test.

Which should you use? Always use SEER2 when comparing new systems (2023+). If comparing to an older system, convert: SEER2 ≈ SEER × 0.95

Current SEER2 Standards

• Northern states: Minimum SEER2 13.4
• Southern/Southwest states: Minimum SEER2 14.3
• High-efficiency systems: SEER2 16-18
• Premium systems: SEER2 19-24+

When SEER Matters Most

SEER is the most important rating when:

• You use AC for extended periods (1,000+ hours annually)
• You experience moderate temperature variations (not constantly extreme heat)
• You want to estimate annual cooling costs
• You're comparing central air conditioners or heat pumps

EER: Energy Efficiency Ratio

What EER Measures

EER measures how efficiently a system cools at a single, specific outdoor temperature: 95°F.

The formula:

EER = Cooling Output (BTU/hr) ÷ Power Input (Watts)

Example: A unit producing 36,000 BTU/hr while consuming 3,000 watts has an EER of 12.

How EER Differs from SEER

EER2: The Updated Standard

Like SEER2, EER2 uses updated testing procedures (2023+):

• More realistic airflow and static pressure
• Typically 3-4% lower than old EER for same unit
• Conversion: EER2 ≈ EER × 0.96

Why EER Matters

EER becomes more important than SEER in specific situations:

1. Consistently Hot Climates

Phoenix, Las Vegas, South Texas, Central Valley California—anywhere temperatures regularly exceed 95°F for extended periods. In these areas, peak efficiency matters more than seasonal average.

2. Commercial Buildings

Commercial HVAC often uses EER because buildings run AC year-round at consistent loads.

3. Window/Portable Units

Room air conditioners are typically rated by EER rather than SEER since they're used more sporadically.

SEER to EER Relationship

Generally, EER ≈ SEER × 0.875

Examples:

• SEER 16 ≈ EER 14
• SEER 20 ≈ EER 17.5
• SEER 24 ≈ EER 21

Variable-speed systems often have better SEER relative to EER because they excel in part-load conditions. Single-stage systems have similar SEER and EER ratios.

When to Prioritize EER

Choose a system with high EER (relative to SEER) if:

• You live in a desert climate (Phoenix, Palm Springs, Las Vegas)
• Summer temperatures consistently exceed 95°F
• You run AC near maximum capacity frequently
• Peak electricity rates are significantly higher than average

HSPF: Heating Seasonal Performance Factor

What HSPF Measures

HSPF measures how efficiently a heat pump heats your home over an entire heating season.

The formula:

HSPF = Total Heating Output (BTU) ÷ Total Energy Input (Wh)

Example: A heat pump that provides 40,000 BTUs of heating while consuming 3,500 watt-hours has an HSPF of 11.4.

Why Heat Pumps Need a Separate Rating

Heat pumps both cool and heat. SEER measures cooling efficiency, but heating is a different process with different efficiency characteristics. Hence, HSPF.

How HSPF Is Tested

Testing simulates a heating season in Region IV (moderate climate):

• Temperature range: 5°F to 62°F outdoor temps
• Indoor conditions: Maintained at 70°F
• Includes defrost cycles: Accounts for efficiency loss during defrost
• Backup heat: Accounts for electric resistance heat when heat pump can't keep up

HSPF2: The New Standard (2023+)

HSPF2 replaced HSPF with updated testing:

• Based on colder Region V (Northern climate) conditions
• More realistic airflow and static pressure
• Better accounts for real-world ductwork
• Typically 15% lower than old HSPF: HSPF2 ≈ HSPF × 0.85

Current minimums:

• Northern regions: HSPF2 7.5
• Southern regions: HSPF2 6.7

HSPF Performance Tiers

• HSPF2 7.5-8: Minimum standard, basic efficiency
• HSPF2 9-10: Good efficiency, suitable for moderate climates
• HSPF2 10-11: High efficiency, works well in colder climates
• HSPF2 11+: Premium cold-climate heat pumps

When HSPF Matters Most

HSPF is critical if:

• You're using a heat pump as your primary heating source
• You live in a cold climate (winter temps regularly below 40°F)
• Electricity is your only heating fuel
• You want to eliminate gas/oil heating

Cold Climate Considerations

Standard heat pumps lose efficiency as temperatures drop. At 5°F, capacity may drop 40-50%. Look for:

• Cold-climate heat pumps: Rated for operation to -15°F or colder
• Variable-speed compressors: Maintain better low-temp performance
• Hyper-heat technology: Specialized designs for extreme cold

In very cold climates (Zone 6-7), even high-HSPF2 heat pumps may need backup heat sources.

COP: Coefficient of Performance

What COP Measures

COP measures instantaneous efficiency—how much heating or cooling you get per unit of energy at a specific moment.

The formula:

COP = Useful Heating/Cooling Output ÷ Energy Input

Unlike SEER and HSPF (which use BTUs and watt-hours), COP is a pure ratio using the same units for input and output.

Why COP Matters

COP is commonly used for:

• Geothermal heat pumps: COP is the standard metric
• Scientific comparisons: Universal measurement (works globally)
• Engineering analysis: Precise efficiency at specific conditions
• Mini-split systems: Many manufacturers use COP

Converting Between COP and SEER/HSPF

SEER to COP (approximate):

Average COP ≈ SEER ÷ 3.41

• SEER 16 ≈ COP 4.7
• SEER 20 ≈ COP 5.9

HSPF to COP (approximate):

Average COP ≈ HSPF ÷ 3.41

• HSPF 10 ≈ COP 2.9
• HSPF 12 ≈ COP 3.5

What Makes a Good COP?

For cooling:

• COP 3-4: Standard efficiency
• COP 4-5: High efficiency
• COP 5+: Premium efficiency

For heating:

• COP 2-2.5: Basic heat pump
• COP 2.5-3.5: Good heat pump
• COP 3.5-4.5: High-efficiency or geothermal
• COP 4.5+: Premium geothermal

Why heating COP is lower: Heat pumps work harder when the temperature difference is greater. In winter, you're trying to move heat from 20°F outside to 70°F inside (50° difference). In summer, you're moving heat from 70°F inside to 95°F outside (25° difference).

AFUE: Annual Fuel Utilization Efficiency

What AFUE Measures

AFUE measures how efficiently a furnace or boiler converts fuel (gas, oil, propane) into heat over a full heating season.

The formula:

AFUE = (Heat Output ÷ Fuel Energy Input) × 100

AFUE is expressed as a percentage. An AFUE of 95% means 95% of the fuel's energy becomes heat, and 5% is lost (mostly through exhaust).

AFUE Performance Tiers

• 50-70% AFUE: Old furnaces (pre-1992), very inefficient
• 78-80% AFUE: Minimum legal standard (non-weatherized natural gas furnaces)
• 90-95% AFUE: Mid-efficiency, condensing furnaces
• 95-98.5% AFUE: High-efficiency, fully modulating furnaces

Why AFUE Differs from SEER/HSPF

AFUE is simpler because:

• Furnaces don't have variable efficiency like heat pumps (they burn fuel consistently)
• No electrical-to-thermal conversion complexity
• Direct fuel-to-heat measurement

When AFUE Matters

AFUE is relevant only if you're using:

• Natural gas furnace
• Propane furnace
• Oil furnace
• Gas/oil boiler (for radiant heat)

If you have a heat pump, ignore AFUE—use HSPF instead.

Which Rating Should You Focus On?

Real-World Efficiency vs. Rated Efficiency

Why Actual Performance Varies

Your system's real-world efficiency will differ from rated numbers due to:

Installation factors:

• Refrigerant charge: ±10% efficiency if incorrect
• Airflow: Undersized ducts reduce efficiency 15-25%
• Ductwork leaks: Can waste 20-40% of conditioned air
• Thermostat placement: Poor location causes short-cycling

Maintenance factors:

• Dirty filters: Reduce efficiency 5-15%
• Dirty coils: Reduce efficiency 20-30%
• Low refrigerant: Drops efficiency significantly

Usage factors:

• Extreme temperatures: Efficiency drops when conditions exceed test ranges
• Humidity: High humidity increases latent load
• Frequent adjustments: Constant thermostat changes reduce efficiency

Maximizing Real-World Efficiency

1. Hire quality installers: Proper installation is worth paying for
2. Get Manual J load calculations: Ensure correct sizing
3. Seal ductwork: Test and seal all duct connections
4. Change filters monthly: Especially during heavy-use seasons
5. Annual professional maintenance: Keeps system at peak efficiency
6. Use programmable thermostats: Reduce unnecessary runtime
7. Maintain steady temperatures: Avoid large setbacks/setups

The Bottom Line

For most homeowners:

• Focus on SEER2 for air conditioning
• Focus on HSPF2 for heat pump heating
• Focus on AFUE for gas furnaces
• Consider EER2 as a secondary factor in hot climates

Don't get overwhelmed by acronyms. These ratings exist to help you make informed decisions. Higher numbers mean lower operating costs—but balance efficiency with upfront cost and payback period.

Use our calculator to see how different SEER ratings translate to real dollar savings in your specific situation.