A few months ago, we did a deep dive into HSPF2 and other key performance metrics on the heating side of HVAC heat pump systems. Now that North America is in a full-blown summer, let's turn our attention to their complementary performance metrics on the space cooling side: EER, EER2, SEER, and SEER2.

In the below article we will:

1. Decode the acronyms.
2. Explain how they are calculated.
3. Provide simple rules of thumb for converting metrics between them.
4. And dive into the Heat Pump Review HVAC dataset to analyze the market and better understand what to expect when buying a new HVAC system.

Calculations & Decoding the Acronyms

Decoding the Acronyms

Before we continue, let's decode the acronyms we'll be discussing:

• EER = Energy Efficiency Ratio
• EER2 = is simply a more strictly defined test procedure for calculating an “Energy Efficiency Ratio”
• SEER = Seasonal Energy Efficiency Ratio
• SEER2 = is a more strictly defined test procedure for calculating a “Seasonal Energy Efficiency Ratio”
• COP = Coefficient of Performance

EER - Energy Efficiency Ratio

The most basic unit in measuring cooling performance is EER, or the Energy Efficiency Ratio. The Energy Efficiency Ratio divides the amount of cooling a machine can generate (measured in BTU) by the energy required to create that cooling as measured by Watts. But wait (you might ask), I thought BTU (British Thermal Units) was a measurement of heat! Indeed, when applied to a cooling system think of the BTU as a measurement of how much heat is removed from the space using the heat pump system.

EER in Energy Star

As we learned in our deep dive on heating measurements, when you see a reported measurement like EER there are still several questions to ask: principally, what temperature was it outside when the measurement was taken? For the purposes of reporting to the US government's “Energy Star” program, EER required testing to occur at an outdoor temperature of 95˚ F, and an indoor temperature of 80˚ F at 50% relative humidity.

EER2

Just like with heating, the US Dept of Energy retired EER for EER2 in January 2023, requiring heat pump and AC-only manufacturers to use more realistic scenarios when taking their efficiency measurements. Again, these systems never work just in isolation, and are either plugged into a ducted system or a mini-split head or other indoor unit. When doing so, static pressure is added and the work of blower motors must be considered in the over all denominator of the equation (energy input in Watts).

SEER & SEER2 - Enter Seasonality

With an understanding of EER and the move to EER2, it can be easy to understand SEER and the move to SEER2.

Whenever you see an “S” in a performance measurement, it likely indicates that the measurement takes into account “ Seasonality .” This seasonality component is important, because it is not always 95˚F outside (in the case of cooling) or 5˚ F in the case of the equivalent non-seasonal “COP” measurement used in heating.

So, SEER and the updated SEER2 both incorporate definitions of seasonality into the texting mix, trying to better represent the type of performance a system would see throughout a cooling season (both when it could be cooler outside than 95˚ F and the system would be more efficient, and when it could be hotter than 95˚ F and the system would be a lot less efficient).

Similar to the difference between EER and EER2, the “2” at the end marks the new standards set by the US Dept of Energy in January 2023 to create a more realistic testing scenario using a higher static pressure due to ducts and the effort the blower motor and other components need to do in order to function as a whole system.

Converting EER, SEER, EER2, and SEER2

Old to New

A lot of people want to convert the older measurements into the newer measurements so they can think about what type of efficiency gain they will see when upgrading from, say, a SEER 12 system to a SEER2 of 18. If they were the same metric you could easily say that 18 would use ⅔ the electricity of the old system. But with the stricter standards, the picture is actually even better.

The below table was developed by the California Energy Commission and provides conversion metrics between SEER and SEER2 that can be helpful depending on what type of system you are comparing. You can see that, for example, the larger the system (e.g. row 2) the greater the difference due to the amount of ducting and the size of the blower motor that would be required to move air through the system. If you aren't sure about which system you have, a conversion of 1.05 seems appropriate (e.g. if you had a SEER 10 before, it was as if you had a 9.5 SEER2).

System Type	Equation
Split system air conditioner < 45,000 Btu/h	SEER = SEER2 X 1.049 EER = EER2 X 1.043
Split system air conditioner ≥ 45,000 Btu/h	SEER = SEER2 X 1.051 EER = EER2 X 1.045
Split system heat pump	SEER = SEER2 X 1.049 HSPF = HSPF2 X 1.173
Packaged air conditioner and heat pump	SEER = SEER2 X 1.045 EER = EER2 X 1.038 HSPF = HSPF2 X 1.176
Space constrained air conditioner	SEER = SEER2 X 1.026
Space constrained heat pump	SEER = SEER2 X 1.008 HSPF = HSPF2 X 1.175
Small duct high velocity system	SEER = SEER2 X 1.000 HSPF = HSPF2 X 1.180

Table: Equations to Convert Respective Ratings for the 2019 Energy Code
Source: California Energy Commission

EER2 to SEER2 Conversion

Given EER2 is performance at an outdoor temperature of 95˚ F, SEER2 is naturally going to be a higher number (because at lower temperatures the heat pump will require less energy to achieve the target indoor temperature, having an easier time “rejecting” heat outdoors).

There also isn't quite as a consistent conversion from EER2 to SEER2 as there was for (S)EER to (S)EER2 because seasonality of outdoor temperature doesn't impact the static pressure of ducting in a dramatic way model to model, but seasonality effects overall performance significantly quite a bit model to model. This is because heat pump manufacturers make many tradeoffs in their designs so it can perform better in various conditions. These tradeoffs include which refrigerant to use, whether the compressor and blower are variable-speed or multi-speed, and the style of the heat rejection coils on the condenser. The most common trade-off is with “Cold Climate” heat pumps, which might be tuned to work best in colder environments at the cost of being less efficient on the hottest days than a system designed more to be performant during the summer.

The below graphs show the relationship between EER2 and SEER2 of all ducted and ductless heat pump systems tracked by Heat Pump Review . In general, we see an average ratio of 1.61 SEER2 to EER2 for ductless systems, and 1.67 for ducted systems , demonstrating that operating at lower outdoor temperatures than 95˚ F makes these systems significantly more efficient.

(interactive versions of these charts are on our HVAC Data Stories page)

What is a “Good” EER2 & SEER2 and What to Buy

Now that we understand EER2 and SEER2 ratings, what should you buy?

As with HSPF2 / heating standards, the first thing you should know are the minimum standards for a system to qualify for an Energy Star rating. Energy Star is important for several reasons, firstly because many state and local rebate programs require a machine to meet Energy Star's requirements, and anything lower will both be worse for the environment and your wallet over the long-term (something might seem cheeper up front, but will consumer a lot more energy).

As of January 2023, Energy Star's new minimum cooling standard is a SEER2 of 15.2 for Heat Pump systems and an EER2 rating of 11.7 (since a heat pump system also heats, it also has to meet the 8.5 HSPF2 heating efficiency marker).

The Inflation Reduction Act (IRA) offers even more savings beyond whatever your local government provides (up to 30% of project costs, with a maximum credit of $2,000) if you meet slightly stricter standards: SEER2 ≥ 16, and EER2 of ≥ 12 for a standard heat pump, or ≥ 9 for a “Cold Climate” heat pump (where the HSPF2 standard is higher, and therefore a performance tradeoff is accepted in cooling).

Beyond the Minimums

As seen in the above graphs, these days there are many options well above these minimum performance standards, and improving the performance will both be better for the climate and save you money over the long-term ownership of your new system by using less energy.

From a review of Heat Pump Review HVAC data , over half of Energy Star certified heat pump systems have an ≥ 18 SEER2 and a full 2,880 systems (32%) are SEER2 of 20 or above, indicating that there are plenty of options to get very high performance.

With so many options, Heat Pump Review 's recommendation is that you ask your installer to quote you a system with ≥20 SEER2.

SEER2 and Ducted vs Ductless

As we have covered before, there will almost always be an efficiency hit when you have a ducted HVAC system vs one of the newer ductless “mini-split” systems. Ducted systems are less efficient due to 2 main reasons: 1) significant electricity is used to blow the conditioned air throughout extensive duct work - duct work that by nature of being restricted space has “static pressure” that makes the blower work harder. This is in comparison to a mini-split system where the blower just blows into the room you want conditioned, with almost no static pressure of note because there's nothing retricting its movement. 2) While the newly conditioned air travels from the central air handler to the room you want heated/cooled, that air may heat up (in the summer) or cool down (in the winter) while traveling in the ducts, making you need to condition even more air. Also ducts - especially older ones - leak up to ⅓ of the air into the unconditioned space it travels through; for example: your ceiling, walls that run along the exterior, or - even worse sometimes - the attic.

In an analysis of the Heat Pump Review HVAC dataset, we found that overall ducted systems have 6.1% lower SEER2 ratings than their ductless mini-split counter-parts. But honing our query a bit more and looking only at like-systems (we compared all 3 ton systems) we see that ducted systems have an even more pronounced performance hit, with ductless mini-splits using 12.1% less energy for the same cooling capacity (avg of SEER2 17.2 vs 19.5).

Conclusion

We hope this article was helpful in better understanding these important cooling performance metrics as you make your HVAC heat pump or air conditioner purchase. If you have any additional questions or data you would like to see added to this article, feel free to contact us as we will update the article as the data changes and new questions are submitted.