What Is Efficiency, Really?
The efficiency of a heat pump can be expressed through several performance indicators, the most common being the Coefficient of Performance (COP) and the Seasonal Coefficient of Performance (SCOP). Both are essential when selecting and sizing the right system for your home.
Coefficient of Performance (COP)
A heat pump only works because it moves heat from a low-temperature source — the ground, outside air, or water — into your home. Moving heat requires energy, and that energy is supplied by electricity. The ratio between the useful heat delivered and the electrical energy consumed is what efficiency is all about.
Where Qh is the thermal energy delivered to the building (in kWh) and W is the electrical input (in kWh). The power drawn from the grid is what the system consumes, so the COP essentially tells you how many units of heat you get for every unit of electricity you pay for.
This COP value represents a snapshot in time — a single moment where the system is operating under defined temperatures, flow rates, and load. In reality, that number shifts constantly. For air-source heat pumps, the COP is heavily influenced by the outdoor air temperature, while for geothermal systems it depends on the ground loop fluid temperature. The warmer the source and the cooler the demand, the higher the COP.
Where Th is the temperature of the heat sink (your indoor space, in K) and Tc is the temperature of the heat source (in K).
Because of the thermodynamic principles above, the lower the temperature gap between the source and the sink, the higher the COP will be. This is why geothermal systems — which tap into stable underground temperatures — typically outperform air-source units during harsh winter months, when outdoor air temperatures plunge and the gap widens significantly.
For installations like Qh and W, what matters is the temperature at the heat pump side of the entering and exiting streams, not the raw ground or air temperature. There will always be a small temperature drop across the heat exchanger, and this difference must be built into the design process. For instance, if the ground loop delivers fluid at 5°C to the heat pump's evaporator, the unit might actually see 3°C on its primary side after exchanger losses are counted. Similarly, on the condenser side, if the building's heating circuit runs at 40°C, the refrigerant side may reach closer to 43°C or 44°C. These gaps always favor the builder of the system — they reduce the theoretical ceiling of efficiency, and that's why proper sizing matters so much.
Seasonal Coefficient of Performance (SCOP)
The COP describes the efficiency of the heat pump at a single set of operating conditions, but since outdoor temperatures vary throughout the year, the COP changes significantly with the seasons. That's why the Seasonal Coefficient of Performance (SCOP) was introduced. It can be calculated as:
Here, SCOP is the energy delivered over the full heating season divided by the total electrical energy consumed during that same period. Unlike a snapshot-style COP, the SCOP gives you a more realistic picture of how the unit will perform across an entire season. In practice, SCOP values are higher than a worst-case COP, because a large share of the heating demand occurs on mild days, not the coldest ones.
There are ideal pumps that do not require any electricity to run, but these are hypothetical devices. Every real heat pump must have compressor input to move heat from a cold source to a warmer sink. The compressor consumes electricity, and its efficiency decreases as the temperature gap between the source and sink grows. Below a certain threshold — typically when outdoor air drops near or below 0°F for air-source units — the COP can fall low enough that resistance heating takes over. Together with thermal losses from the compressor, this means more energy is drawn from the grid to deliver the same heat.
Because these numbers change from season to season, the benchmark most often used on equipment labels is the SCOP, not the raw COP. It represents the blended average across the entire heating period, weighted by how many hours the system runs at each outdoor temperature. When comparing units, always look at the SCOP figure first — it gives you a far better picture of real-world operation than any single snapshot.
Energy Efficiency Ratio (EER) and Seasonal Energy Efficiency Ratio (SEER)
The paragraphs above discussed the efficiency of a heat pump during heating, but a similar concept can be used for the cooling mode. Here, the terminology is Energy Efficiency Ratio (EER) defined as follows:
Where Qc is the energy extracted from the building, expressed in BTU/hr, and W is electrical input in watts.
The energy ratio during cooling (Qc / W) is called EER. Similarly, the SEER is defined as:
Similar to the heating mode, also a theoretical EER can be calculated as follows:
The terms Qh, Th, Qc and Tc relate to the hot and cold sides of the heat pump. Therefore, when a heat pump is in cooling mode, the Qh is the energy extracted from the ventilated home and Qc is the energy emitted to the building at a higher temperature. During cooling, the cooling number as the one supplied at cooling is the dominant one. SCOP has the role of the efficiency of the air handler unit.
If your heat pump is modulating, then the total SEER value is typically slightly higher than the snapshot EER because of lower average capacity demand and shorter running hours outside of peak. When comparing systems, always consider the SEER numbers over the simple EER when making your purchase decision for long-term use.
Modulating Heat Pumps
Although heat pumps may appear similar on the surface, there can be significant differences on the inside. One key difference lies between on/off and modulating heat pumps.
With an on/off heat pump, your system is always either fully on or fully off. As the name suggests, the unit turns on when the building calls for heat, delivers its full load, and then shuts off to cool down again. Modulating units are smarter — they adjust the compressor output based on actual demand, ramping up when the house is cold and scaling down to just barely sip power when conditions are mild. That translates into fewer start/stop cycles, less wear on components, and noticeably more stable comfort.
If your heat pump is modulating, it can operate at 100 percent of its capacity, but only at 30 percent or less of its nominal output. Running at lower capacity means the unit's compressor works less, which increases efficiency and extends the lifespan of the system. However, it's important to note that it's not always true that a lower load translates to higher efficiency — the pump must still operate within its efficient band. Modulating systems are designed to find that sweet spot and stay there.
When a modulating heat pump is working at a reduced regime (e.g. lower than maximum capacity), there is another advantage. The maximum temperature difference between the heat pump's two heat exchangers shrinks, because the entering and leaving fluid temperatures move closer to the supply and return design values. In other words, a smaller lift for the compressor means the whole Carnot cycle tightens up, and the system consumes less power.
For completeness, on/off heat pumps can also have a form of modulation. For example, some heat pumps with a slightly bigger capacity (for example, 50 kW) could be split into two modules each with 25 kW capacity, since these modules can turn on and off separately. Together, they can provide roughly 8 intermediate levels, but even smaller and smarter full-modulating units offer a far more granular response. These are referred to collectively as stage-based or multi-speed heat pumps, but full modulation still delivers the most efficiency over the long run.
Efficiencies In Borefield Design
When you are designing a borefield with a cooling load, a good approach is to aim as high as possible to what the ground will allow, the key goal being that a high SCOP and SEER will signify that the installation will be efficient. These are the two most important values when sizing the heat pump. A high SCOP will lower your heating cost, and a high SEER means cooling costs are reduced too. But a ratio of those two (sometimes the system is heating-dominated or cooling-dominated) can be the most powerful indicator of how the borefield should be shaped.
Using SCOP
Using the SCOP at the source side of the heat pump, you can calculate the needed ground heat load. Since the building load is known, the ground load depends on how efficient the pump is at converting electricity into heating capacity. A higher SCOP means less demand placed on the borefield, which lowers installation cost and maintenance requirements.
Using SEER
The efficiency of a heat pump depends on the fluid temperature coming out of the borefield. So a high SEER in cooling is only possible when the ground loop stays cool. If your borefield is too small, temperatures will creep up, and the SEER will drop over the years. Balanced loading and smart design protect this efficiency across decades of use.
The efficiency of a heat pump depends on the temperature coming out of the borefield, so the heat pump design and the borefield design are tightly coupled. When sizing the two, always treat them as a single system: a great heat pump on an undersized borefield will never deliver its rated numbers.
It should also be clear that, because the efficiency of the heat pump directly depends on the fluid temperature leaving the borefield, better system design raises efficiency over the long run. The cumulative fuel savings over 20 years can easily pay back the small incremental cost of an oversized borefield. Ultimately, efficiency is not only about choosing the right heat pump — it's about choosing the right system around it.
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Shop Armstrong Air 14 SEER Heat PumpConclusion
In this first chapter, we covered the different efficiency metrics — COP, SCOP, EER, SEER — and why they matter so much when choosing a heat pump. We also looked at how modulating technology lifts efficiency even further, and how it all feeds directly into the borefield design. In the next chapter, we'll dive into how those ground temperatures shift over time, and how borefield geometry influences long-term system performance.
Quick Review Questions
- By looking at the formula for COP in heating mode, can you tell under which conditions the heat pump operates most efficiently? Is it better to supply a building with a low-temperature radiant system or a high-temperature fan coil?
- The COP of a heat pump at a given operating point is always higher than its SCOP over the full heating season. True or false? Justify your answer.
- Heat pumps with modulation always produce a higher SCOP than on/off units. True or false? Justify your answer with a short example.