Geothermal Heat Pump Efficiency Ratings Explained: EER, COP, SEER2 & HSPF2 (2026 Guide)

By Geothermal Insider · Updated March 25, 2026

Shopping for a geothermal heat pump? You'll see a blizzard of acronyms: EER, COP, SEER2, HSPF2, and sometimes older ratings like SEER and HSPF. Manufacturers throw these numbers around like they're self-explanatory, but unless you've got an engineering degree, they're anything but.

The short version, before anything else: geothermal heat pumps are rated in EER and COP. SEER2 and HSPF2 belong to central air conditioners and air-source heat pumps. Those are the two numbers you'll actually be comparing on a geothermal quote, and mixing them up with the air-source metrics is the single easiest way to talk yourself into the wrong system.

Here's the plain-English guide to what these numbers actually mean, how they translate to real dollars on your energy bill, and which ones matter most when you're comparing systems.

Why Efficiency Ratings Matter

A geothermal heat pump's efficiency rating tells you how much heating or cooling you get for every dollar of electricity. The difference between a "good" and "great" rating isn't trivial — it's hundreds of dollars per year over a system that lasts 20–25 years.

Here's a quick example. Take a home with a 60 MMBtu annual heating load, assuming electricity at 16¢/kWh. At COP 3.5 that heat costs about $800/year; at COP 4.5, about $625 — a difference of roughly $180 per year. Double the load to 120 MMBtu (a large home in a cold climate) and the same COP gap is worth about $355/year. Over a 20-year equipment life that's $3,600 to $7,100.

Note what that means: the value of a higher COP scales directly with how much heating you actually buy. The same efficiency upgrade that pays for itself in Minnesota may not in Georgia. Run the numbers against your own load, not a generic figure.

But here's the catch: different ratings measure different things, under different conditions. You can't just compare numbers across rating types. An EER of 20 and a COP of 4.0 are measuring completely different aspects of performance.

Every Rating at a Glance

EER: The Cooling Snapshot

Energy Efficiency Ratio (EER) measures how efficiently a heat pump cools at a single set of conditions — specifically, 95°F outdoor temperature (or for geothermal, a specific entering water temperature), 80°F indoor, and 50% humidity.

How to read it: EER = BTU of cooling output ÷ watts of electricity consumed. An EER of 20 means the unit produces 20 BTU of cooling for every watt-hour of electricity. An EER of 30 means 30 BTU per watt-hour — 50% more efficient.

Why it matters for geothermal: EER is measured at a single test point, which makes it useful for comparing one unit to another under identical conditions. But real-world conditions change constantly — entering water temperature varies by season and geography, and your cooling load fluctuates throughout the day. It's also the right metric for cooling, and our geothermal cooling guide explains why EER beats SEER for judging a ground-source system.

Geothermal vs. conventional: This is where geothermal shines. A conventional air-source heat pump typically has an EER of 10–15. A mid-range geothermal unit delivers EER 20–25. A premium variable-speed geothermal system can hit EER 30–45. That's 2–3× more cooling per dollar of electricity.

COP: The Heating Snapshot

Coefficient of Performance (COP) is the heating equivalent of EER. It measures how much heat a system delivers compared to the electricity it consumes, at a single set of conditions.

How to read it: COP is a simple ratio. A COP of 4.0 means the system delivers 4 units of heat for every 1 unit of electricity consumed — effectively 400% efficient. This isn't magic; the heat pump isn't creating energy from nothing. It's moving heat from the ground into your home, and the ground is doing most of the heavy lifting.

The baseline comparison: An electric resistance heater (baseboard, space heater, electric furnace) has a COP of exactly 1.0 — it converts electricity to heat at 100% efficiency. A gas furnace at 95% AFUE has an effective COP of about 0.95 for gas-to-heat (though the comparison isn't perfectly apples-to-apples since gas and electricity have different costs). A geothermal system at COP 4.0 is delivering 4× more heat per unit of energy input than electric resistance.

*Assuming electricity at 16¢/kWh — a round figure used for illustration. The U.S. average residential rate was 17.30¢/kWh for 2025 (EIA, Electric Power Monthly Table 5.3), and state rates range from roughly 12¢ to over 40¢. Scale these figures to your own rate. 1 kWh = 3,412 BTU.

The geothermal COP advantage over air-source heat pumps: Air-source heat pumps lose efficiency as outdoor temperatures drop — COP can fall to 1.5–2.0 at 0°F. Geothermal systems don't have this problem because they draw heat from the ground (a constant 45–65°F), not the frigid outdoor air. A geothermal system maintaining COP 3.5 at -10°F outdoor air is vastly outperforming an air-source unit struggling at COP 1.8. This is why geothermal is the superior choice in cold climates.

SEER2: The Seasonal Cooling Picture (Air-Source Only)

Seasonal Energy Efficiency Ratio 2 (SEER2) is the newer version of the SEER rating, updated in January 2023 to use more realistic test conditions (the "M1" test procedure with higher external static pressure — basically, testing with the ductwork resistance a real system actually faces).

How to read it: SEER2 represents cooling efficiency averaged across an entire cooling season, accounting for the fact that your system runs at different loads and conditions throughout summer. It's more representative of real-world performance than EER, which measures a single peak condition.

SEER2 vs. old SEER: SEER2 numbers are typically 4–6% lower than the old SEER rating for the same equipment, because the test conditions are harder. A unit rated SEER 20 under the old standard might rate SEER2 18.8 under the new standard. It's not less efficient — it's just measured more honestly.

For geothermal systems: SEER2 is not a geothermal metric. Ground-source equipment is rated in EER under ISO 13256, not SEER2 under the air-source test procedure — the two are measured against completely different heat sinks (ground loop water vs. outdoor air), so a geothermal EER and an air-source SEER2 are not comparable numbers and cannot be ranked against each other. If you are cross-shopping a geothermal system against a high-efficiency air-source heat pump, compare estimated annual operating cost in dollars, not rating to rating.

HSPF2: The Seasonal Heating Picture (Air-Source Only)

Heating Seasonal Performance Factor 2 (HSPF2) does for heating what SEER2 does for cooling — it averages heating efficiency across an entire heating season, under the updated M1 test procedure.

How to read it: HSPF2 is expressed in BTU per watt-hour (like EER), but averaged over a heating season. Higher is better. To convert HSPF2 to an approximate seasonal COP, divide by 3.412.

Example: An HSPF2 of 13 ÷ 3.412 = seasonal COP of approximately 3.8. That means across the full heating season, the system delivers about 3.8 units of heat for every unit of electricity — including startup losses, defrost cycles, and part-load operation.

A caution on that comparison: an air-source HSPF2 is a full-season average that already includes defrost cycles and cold-weather derating, while a geothermal COP is a steady-state figure at one entering water temperature. The geothermal number is measured under friendlier conditions, so knock 10–20% off it (see real-world vs. rated below) before putting the two side by side.

How to Compare Systems Using Ratings

When you're comparing geothermal units, here's how to use the ratings effectively:

Rule 1: Compare the Same Rating Type

EER to EER. COP to COP. Never compare a unit's EER to another unit's COP — they measure different things in different modes.

Rule 2: Check the Test Conditions

Geothermal ratings are published at specific entering water temperatures (EWT), and the EWT depends on the application, not on load. Under ISO 13256, each application has its own paired cooling and heating rating conditions:

• Ground-loop (closed loop): rated at 77°F EWT in cooling, 32°F EWT in heating
• Ground-water (open loop): rated at 59°F EWT in cooling, 50°F EWT in heating
• Water-loop (building loop): rated at 86°F EWT in cooling, 68°F EWT in heating

Note that 77°F and 32°F are not "full load" and "part load" — they are the cooling and heating rating points for a closed-loop system. That is why a closed-loop unit's EER is quoted at 77°F and its COP at 32°F: they are different modes, not different loads.

A unit might have a COP of 4.5 at 50°F EWT (open loop) but only 3.2 at 32°F EWT (closed loop). Both are valid — they just reflect different applications. When comparing units, make sure you're comparing at the same EWT and the same loop type, and never compare an open-loop rating to a closed-loop one.

Rule 3: Weight Heating vs. Cooling for Your Climate

Rule 4: Weigh the Variable-Speed Premium Against Your Own Numbers

Variable-speed (inverter-driven) compressors generally outperform single-speed and two-stage models, and the advantage is largest at part load — which is where a correctly sized system spends most of its running hours. That is a real benefit, but it is not a clean sweep of every rating: a given two-stage unit can match or beat a given variable-speed unit at a specific full-load rating point. Compare the actual spec sheets for the two models you're choosing between rather than assuming the variable-speed unit wins every line.

The premium typically runs $2,000–$5,000. Whether it pays back depends entirely on how much heating and cooling you buy each year, so run it against your own bill rather than a generic figure: divide the quoted premium by the annual dollar savings the installer projects for the two units. On a home spending $900/year on geothermal operating cost, a 20% efficiency gain saves $180/year — a $3,000 premium takes about 17 years to return. On a home spending $2,000/year, the same 20% saves $400/year and the premium returns in about 8 years. Bigger loads justify the upgrade; small loads often don't.

For more on specific brands and models, see our best geothermal heat pump brands guide. For a model-by-model read on the two brands homeowners cross-shop most, with every published rating shown alongside the loop type and entering-water temperature that produced it, see our WaterFurnace review and ClimateMaster review.

Real-World vs. Rated Efficiency

Lab ratings don't lie, exactly — but they don't tell the whole truth either. Real-world efficiency depends on factors the lab test can't replicate:

Loop field design — An undersized loop field delivers warmer water in summer and colder water in winter, reducing both EER and COP. A well-designed loop maintains the entering water temperatures the unit was rated for. This is why proper sizing matters so much.

Ductwork condition — Leaky ducts waste 20–30% of the system's output. A unit rated at COP 4.5 effectively operates at COP 3.2 if a third of its output leaks into your attic. This isn't the heat pump's fault — it's a distribution problem, and it's why the whole geothermal HVAC system has to be judged together rather than by the unit's nameplate rating alone. See our retrofit guide for ductwork assessment.

Thermostat behavior — Frequent setback/recovery cycles (dropping temp at night, cranking it up in the morning) force the system to work harder than steady-state operation. Variable-speed systems handle this much better than single-speed.

Ground conditions — Local soil thermal conductivity directly affects loop performance. Sandy, dry soil (low conductivity) means the loop field works harder. Clay or saturated soil (high conductivity) means better performance. A soil conductivity test can predict real-world loop performance before installation.

Rule of thumb: Expect real-world efficiency to be 10–20% lower than rated peak efficiency. A COP 4.5 system will likely average COP 3.6–4.0 across the heating season. That's still dramatically better than any combustion system.

Minimum Standards and ENERGY STAR

Two things people commonly get wrong about this table.

First, these are ENERGY STAR certification thresholds, not a legal minimum to sell. ENERGY STAR is a voluntary EPA labeling program. A unit that falls below 17.1 EER can still be sold in the United States; it just can't wear the label. Federal energy conservation standards for commercial water-source heat pumps sit separately in 10 CFR Part 431, and they are a different set of numbers for a different class of equipment.

Second, the classes are not interchangeable. There is no single "geothermal minimum" — a water-to-water open-loop unit is held to 20.1 EER while a water-to-air closed-loop unit is held to 17.1, and a DGX system is measured under an entirely different standard. Make sure any efficiency claim you're handed names the product class it applies to.

On ENERGY STAR Most Efficient: EPA currently recognizes all ENERGY STAR certified geothermal heat pump models as Most Efficient, rather than carving out a higher-performing subset the way it does for some other product categories. So "ENERGY STAR Most Efficient" is not a tie-breaker between two certified geothermal units — if both are certified, both carry it, and you'll need to compare their actual EER and COP figures instead.

Important note: The 30% federal tax credit under IRC §25D is no longer a factor in this decision. It applied only to property placed in service through December 31, 2025 and is not available after that date. While it existed, it applied to all ENERGY STAR-qualified geothermal systems equally, on total installed cost — a $40,000 premium system got $12,000 back and a $25,000 standard system got $7,500 — so it rewarded total investment rather than incremental efficiency. For a 2026 install there is no federal credit on either, which means the efficiency-tier decision now rests entirely on the energy savings it buys you. Some state and utility programs do tier their rebates by efficiency; check your state guide.

What to Prioritize When Shopping

After 20 years of designing geothermal systems, here's what I'd prioritize if I were buying one today:

1. Installer quality over equipment brand — A great installer with a mid-range unit will outperform a mediocre installer with a premium unit every time. The loop field design and ductwork matter more than the nameplate COP. How to vet installers →

2. Variable-speed compressor — The single biggest efficiency upgrade. Worth the premium in virtually every climate and application.

3. COP at your local conditions — Don't compare COP at 77°F EWT if your ground temperature is 48°F. Ask the installer what COP to expect at YOUR entering water temperature.

4. Desuperheater — Adds roughly $500–$800 to the install and recovers waste heat from the compressor to help heat your domestic hot water. How much it saves is highly case-specific: it produces the most hot water when the system is running hardest, which may not line up with when your household actually uses hot water, and it does very little in shoulder seasons when the compressor is idle. Ask your installer to estimate the savings for your household's hot water usage and run the payback yourself rather than assuming a rule of thumb. Our guide to geothermal water heating covers what the device actually delivers and where the widely-quoted coverage figures come from.

5. EER for cooling-dominant climates — If you run AC 6+ months per year (Texas, Florida, Arizona), EER at 77°F EWT is your most important number.

6. Don't chase the absolute highest rating — COP improvements have diminishing returns, because each step up saves a smaller slice of an already-smaller bill. On an 80 MMBtu annual heating load at 16¢/kWh, going from COP 3.0 to 4.0 saves about $315/year; going from COP 4.0 to 5.0 saves only about $190/year for a bigger price jump. The premium for the very top unit often isn't justified — check where your quoted models actually sit before paying for the last increment.

Frequently Asked Questions

Sources

1. ENERGY STAR — Geothermal Heat Pumps Key Product Criteria — EER/COP thresholds by product class, referenced test standards, and the three-phase exclusion (accessed July 2026)
2. ENERGY STAR — Most Efficient Certified Geothermal Heat Pumps — confirms all certified geothermal models are recognized as Most Efficient
3. U.S. Department of Energy — Geothermal Heat Pumps (accessed July 2026)
4. U.S. DOE, Federal Energy Management Program — Purchasing Energy-Efficient Geothermal Heat Pumps
5. ISO 13256-1 (water-to-air) and ISO 13256-2 (water-to-water) — the rating standards ENERGY STAR references for geothermal heat pump EER/COP
6. AHRI Standard 870 — Performance Rating of Direct GeoExchange Heat Pumps. Applies to DGX (refrigerant-in-ground) systems only, not to conventional open- or closed-loop water-source equipment
7. 10 CFR Part 431, Subpart F — federal energy conservation standards for commercial water-source heat pumps. Note this is a separate regime from the ENERGY STAR residential criteria above
8. WaterFurnace — 7 Series and 5 Series performance data sheets (EER/COP at multiple EWTs)
9. ClimateMaster — Tranquility series performance specifications
10. Bosch — Greensource SI and CDi series rated performance data
11. U.S. EIA — Electric Power Monthly, Table 5.3 — U.S. average residential electricity price