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

By Sarah Chen, Energy Policy Analyst · 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.

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: For a 3-ton system in a moderate climate, the difference between a COP of 3.5 and a COP of 4.5 is roughly $200–$350 per year in electricity costs. Over 20 years, that's $4,000–$7,000 — enough to justify the higher upfront cost of a premium unit in most cases.

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.

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.

*Based on electricity at 16¢/kWh. 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

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 ratings for geothermal are less commonly advertised than EER and COP because the industry has traditionally focused on those metrics. But as the DOE pushes standardization, you'll see SEER2 and HSPF2 on more geothermal product listings.

HSPF2: The Seasonal Heating Picture

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.

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). The two standard test conditions are:

• 77°F EWT (full load) — represents summer cooling or warm-climate heating
• 32°F EWT (part load) — represents cold-climate winter heating

A unit might have a COP of 4.5 at 77°F EWT but only 3.2 at 32°F EWT. Both are valid — they just reflect different operating conditions. When comparing units, make sure you're comparing at the same EWT.

Rule 3: Weight Heating vs. Cooling for Your Climate

Rule 4: Variable-Speed Premium Is Usually Worth It

Variable-speed (inverter-driven) compressors consistently outperform single-speed and two-stage models across ALL efficiency ratings. They cost $2,000–$5,000 more upfront but deliver 15–30% better seasonal efficiency. For a system you'll run for 20+ years, that premium pays for itself in 5–8 years.

For more on specific brands and models, see our best geothermal heat pump brands guide.

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. 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

Important note: Unlike air-source heat pumps, where higher SEER2/HSPF2 ratings unlock higher rebates in some programs, the 30% federal tax credit for geothermal applies to ALL ENERGY STAR-qualified systems equally. You don't get a bigger credit for a higher-efficiency unit. The credit is on the total installed cost — so a $40,000 premium system gets a $12,000 credit, while a $25,000 standard system gets $7,500. The credit rewards total investment, not incremental efficiency.

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 $500–$800 and cuts water heating costs 30–50%. The ROI is almost always under 2 years.

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 — The jump from COP 4.0 to COP 5.0 saves about $150–$250/year. The jump from COP 3.0 to COP 4.0 saves $300–$500/year. Diminishing returns above COP 4.0–4.5 mean the premium for the very top unit often isn't justified.

Frequently Asked Questions

Sources

1. U.S. Department of Energy — Geothermal Heat Pumps (accessed March 2026)
2. AHRI Standard 870 — Performance Rating of Direct GeoExchange Heat Pumps (test procedures for EER/COP)
3. DOE 10 CFR 430 — Energy Conservation Standards for Geothermal Heat Pumps (minimum EER/COP)
4. ENERGY STAR — Geothermal Heat Pumps Program Requirements (2026 criteria)
5. ASHRAE 90.1 — Energy Standard for Buildings, ground-source heat pump efficiency tables
6. International Ground Source Heat Pump Association (IGSHPA) — equipment performance standards
7. WaterFurnace — 7 Series and 5 Series performance data sheets (EER/COP at multiple EWTs)
8. ClimateMaster — Tranquility series performance specifications
9. Bosch — Greensource SI and CDi series rated performance data
10. U.S. EIA — State Electricity Profiles (2024 data for cost-per-BTU calculations)