What is Coefficient of Performance (COP)? Simple Explanation & Guide

Alright, listen up. Meaning about what is coefficient of performance, are you interested? Maybe you’ve heard murmurs, glimpsed some numbers — possibly scratched your head. And what’s the big deal with this COP stuff, anyway, and how does it actually shake out when you’re just trying to keep your home at a reasonable temperature and not get absolutely hammered every month on your energy bill?

Let’s cut the crap. “Coefficient of Performance” (COP) sort of translates to how much your system is street in terms of efficiency for things like heat pumps, refrigerators and air conditioners. You could think of it as a rating of how much good stuff (heating or cooling) you get out for a shove of energy you put in — the higher the rating, the better? Higher efficiency. That means less energy consumed, less operating cost. Simple as that. It’s a central idea in thermodynamics, for which that you’ll find a heat-pump whenever you’re moving heat from one place to another.

COP: The True Performance Metric of Your System

So what is it that we mean when we talk about a “score”? COP isn’t some nebulous marketing fluff. It’s a direct ratio. You simply take the “useful heating or cooling produced” – that’s your output – and you divide it by the “work (energy) required” – that’s your input.

Here’s the basic gig:

COP = Output / Input

Simple, right? For a heat pump heating, it is the heat that it delivers compared to the amount of energy it uses. For a refrigerator or air conditioner cooling, it’s the heat that it pulls out versus the energy it uses.

I’m talking about the quantity of thermal energy that’s been moved or delivered, compared to the work or heat we had to put in to make it happen.

Why Your System’s COP Can Flex Higher Than 1

This is where things get interesting, and depending on your way of thinking about “old school” efficiency, maybe a bit counter intuitive, say, a furnace that simply burns some fuel. Furnaces use fuel to generate heat. (If they were 100 percent efficient (which they can’t be) we’d say (1) = “efficiency” of 1. You put one unit’s worth of fuel energy in, you get one unit of heat out.

But heat pumps, air conditioners and refrigerators? They’re different beasts. They not only convert energy; they transport preexisting heat.

Think of it like this:

• It takes a certain amount of energy to create heat from nothing (as in burning fuel).
• Transferring heat that’s already present (in much the same way that pushing air or water) often requires far less energy.

Since these systems are mainly moving heat from one location to another — sucking heat from a cold location and pumping it to a warm locati0n, or doing the opposite — they can provide more heating or cooling power than the electrical energy they use to do the moving.

That’s the cheat code right there. It takes much less work to move heat than to produce it. This is why the COP of these devices can (and often does) exceed \(1\). They are not producing energy where there was none — that would violate the laws of physics. They are just very good at tapping the thermal energy that is already lingering in the environment.

Heating vs. Cooling COP: What’s the Difference?

Now, the COP is not a one-size-fits-all number for a single machine like a heat pump. It also makes a difference if that machine is heating your space versus cooling it.

The math is a little different because depending on the job, it’s the heat flow relative to a different temperature reservoir you’re interested in:

For Cooling (Fridge / Air Con): You Should concentrate on the Heat you’re taking out of the cold place.

• COP_cooling = Heat Removed (from cold) / Work tabcInput
• Or, in notation : COP_cooling = |Q_C| / W

For Heating (Heat Pump): You are concerned with the heat contributed to the hot space. This heat delivery is in fact the heat extracted from the cold source plus the work done on the system.

• COP_heating = Heat Delivered (to hot) / Work Input
• Or: COP_heating = |Q_H| / W

Here’s a neat trick: since the heat delivered for heating is the heat lost from the cold sink plus the work done, and the heating COP is the cooler COP plus 1, the heating COP is usually 1 higher than the cooling COP for the same machine between the same temperature reservoirs.

COP_heating = COP_cooling + 1

So, within a similar operating range, a heat pump that works very well to cool will probably also do an even better job of heating.

Let’s Talk: What Influences Your System’s COP?

If COP is a performance metric, what factors turn performance up or down? This isn’t just spec sheet trivia; it’s critical when knowing how your system will actually work in your home.

The greatest impact on COP by far, as everyone knows, is the operating conditions. Namely, the absolute temperature, and the difference in temperature between where you are taking heat from (the source) and where you are putting it (the sink).

The Foe of Your System: The Temperature Gap (ΔT)

Picture trying to get water to flow uphill. If the hill isn’t so big (small ΔT, perhaps you’re heating a semi-warm house from cool outdoor air), it takes less work. If the hill is a lot (big ΔT, trying to heat a very warm house when the air outside is godawful cold), then this is a hell of a lot harder.

If you want the most energy out of it, you don’t let it cool off and you produce useful energy as long as possible after heating. The delta of temperature (ΔT) the system is fighting against, (in this case how much cooler it gets until it has to top up before another use) will be proportional to the COP. This is a fundamental limit.

• In the case of heating, if the outside is colder while the inside is the same temperature or warmer, a smaller gap is produced, and the COP is lower.
• For cooling, if the outside is warming up with you trying to keep the inside cool, the gap increases and your COP falls.

This explains why the capacity and COP of a heat pump are reduced greatly at a low outdoor temperature.

System Design Matters Too

It is not all temperature. How the system is constructed is a major factor.

• Heat exchangers: Larger, better heat exchangers can facilitate the transfer of heat effectively which in turn can minimize the internal temperature gradient that the compressor has to fight, reflected through higher COP.
• Fluid flow: Pipe and air duct size matters for flow. Larger ones have less resistance (less head loss), less turbulence (quieter!), and the pumps and fans that churn the fluid use less energy. More energy input for the same heat transfer? Higher COP.
• Ground systems: If you have ground-source heat pump, knowing the ground’s thermal conductance and sizing the ground loop properly will let you send back warmer fluid to your system, raising the temperature at which the heat pump may begin operate and therefore overall COP.

What Makes Absorption Chillers So Special?

Just a short post on another taste: absorption refrigerator chillers. Those bad boys don’t depend on a mechanical compressor the way most systems do. They rely on chemical reactions triggered by heat to produce the cooling. Their average COP is usually less than those of compressor systems. But you can bring up their efficiency by stacking stages — such as pairing double- or triple-effect chillers — and actually get their COP above 1.

The “Perfect” COP? (Spoiler: It’s Theoretical)

Just as there is an ideal but never quite achievable efficiency of heat engines based on the Carnot cycle, there is a theory maximum COP for refrigerators (and, by extension, heat pumps). These are the maximum flex numbers that thermodynamics says are possible if a machine was perfect and reversible (like an ideal Carnot heat engine running backward).

This “optimal” COP depends only on the absolute temperatures of the hot and cold heat reservoirs between which the device cycles (using Kelvin temperature):

• Maximum Theoretical COP_heating = T_H / (T_H – T_C)
• Maximum Theoretical COP_cooling = T_C / (T_H – T_C)

Where T_H is the temperature of the hot reservoir and T_C of the cold reservoir.

But let’s get real. Real systems never achieve these numbers. They are a theoretical limit that engineers chase and chase, but never catch. Like chasing a ghost with a washboard abdomen.

Even those fancy thermoelectrics, which generate electricity directly from cooling/heating through the Peltier effect, are bound by a theoretical limit set by figure of merit (zT). Proven materials would push their COP higher, they said, but likely not more than about 40% of ideal, so to get to kind of COPs that standard compressor systems get (like 1.2-1.4 for old Freon units, or 3-5 for entirely new ACs), you’d need to hit zT values (3-4) that have seem virtually no hits so far. So that’s hot for niche spot cooling, though they’re not replacing your home AC quite yet.

Real World Measurement of COP: Examples & Standards

Well, I mean, theoretical limits are great in the textbook right. What about reality? How do we quantify or address COP for real system?

Systems are tested at a set of specific conditions to generate the COP numbers. In Europe, typical test conditions for earth energy systems could be 35°C (308 K) and 0°C (272 K) for the two sides.

Now for those EU ground source figures.

• Theoretical max COP_heating: 308 / (308-273) = 8.8 (C is the temperature in Kelvin)
• Theoretical max COP_cooling: 273/(308-273) = 7.8

Now, the reality check:

• Actual ground source systems tend to be most economical with test results in this range are in the 4.5 range.
• If you look at the seasonal performance, which does include things like pumping energy, it’s closer to 3.5 or less. Still good, but not close to the theoretical max.

Regarding the air source heat pumps, there are other EU standard test conditions: 20°C for hot (indoor) and 7°C for cold (outdoor). The COP you see from such tests may not be indicative of the way heat pumps would perform in real freezing winter weather.

Most off-the-shelf aircons you see zipping around will be between 3.5-5 COP.

As we mentioned, absorption chillers are typically lower, although multistage ones can get around that COP 1 issue.

Locating Your System’s COP: A Little Bit of a Scavenger Hunt

Here’s the thing: Although COP is an incredibly helpful performance parameter, it’s not always necessary for it to be clearly labeled on US residential HVAC systems. You may encounter different sorts of ratings such as SEER, EER, or HSPF but generally not an explicit COP value, especially for heating.

Determining the actual, instantaneous COP typically requires diving into specification data sheets from the manufacturer. It’s your HVAC contractor who usually has the hookup to those kinds of things.

The COP (and heating/cooling capacity) at various outdoor temperatures can be read from these sheets. One example would be with a Trane XR16 3-Ton heat pump it had 3.80 at 47°F outdoor but then it fell to 2.60 at 17°F outdoor (both at 70°F indoor). You can see how that temperature differential gnaws at performance, right?

If you don’t have a spec sheet then you’re pretty much guessing – especially for heating COP. You can convert an EER (Energy Efficiency Ratio), if you have it, into a cooling COP but to do so with HSPF (Heating Seasonal Performance Factor) is less sound since it’s a seasonal average and not an instantaneous one.

Let’s quickly run down those sister ratings:

• EER (Energy Efficiency Ratio): Measures how efficiently a new cooling device operates at a single outdoor/indoor temperature condition (95°F/80°F). You can directly convert EER to COP for cooling by multiplying EER with 0,293.
• HSPF(Heating Seasonal Performance Factor): A measure of the operating cost of a heat pump in the heating season, including the effects of the defrost cycle but not including supplementary heating. It’s closer to seasonal bills but not to an instantaneous COP. You can convert HSPF to a sort of seasonal COP by multiplying by 0.293, but it’s not the same as the instantaneous COP found on a spec sheet.
• SCOP (Seasonal Coefficient of Performance): a newer European rating for heat pumps that provides a more realistic picture of efficiency over an entire year than single point COP. Like SEER/HSPF, an idea was formed, but different calculation.
• SEER (Seasonal Energy Efficiency Ratio): Primarily used for air conditioning, averaged over a cooling season.

(“At the Specific Instance) if you are looking for that instantaneous snapshot. What SCOP, SEER, and HSPF are, are seasonal ratings which indicate the average performance is over time. Both are valuable, but they inform you of different things.

Level Up Your COP: Increasing Efficiency

Ok, so you know what influences COP. So what are some ways to get that number up — while keeping those energy bills down?

The idea here is to minimize the temperature gap (ΔT) that the system must overcome.

For Heating (Heat Pumps):

• Lower the Output Temperature: Rather than blowing devilishly hot air, there are systems that work effectively at lower water or air temps (i.e., piped floor/wall/ceiling heating, oversize radiators). That’s because the heat pump doesn’t need to heat the indoor distribution system as much, thereby lowering ΔT.
• Raising Input Temperature: This is just heating up the source. For the ground source systems, good ground loop or borehole sizing according to correct thermal conductivity leads to warmer fluid extraction from the earth. The temperature of the input can also be increased by accessing things such as solar-assisted thermal banks or using ground water instead of the colder outdoor air.

For Coolant Systems (Air Conditioner / Refrigerator):

• Cooler Source Use: Groundwater, if available, can be used as a heat sink rather than outdoor air.
• Decrease Temp Difference Output: On ACs, increasing the flow of air can decrease the temperature drop required across the output side (distance).

Improve System Design:

• Size Matters (Sometimes): Oversize internal heat exchanges, pipes and air canals. It will decrease resistance, turbulence, and power cost for pumps/fans which helps in increasing total COP. It also adds to the cost, but in theory increases efficiency over the life of the system.
• Precision Ground Work: For ground loops, accurate thermal conductivity testing and sizing are key to achieving warmer return temperatures.

Absorption Chiller Specifics:

• Doubling or tripling the stages (double or triple effect) can substantially increase the COP, up to perhaps greater than 1.

The idea behind increasing COP is that you want to help how easy it is for the system to move heat, ergo less “work” is needed for the “output”.

Seasonal efficiency: The full picture

Instantaneous COP is very handy for understanding performance at a given point in time (or a pseudo-steady-state test condition). But your energy bill is accumulated over time. Enter Seasonal Efficiency.

These statistics, such as for instance SCOP (for heat pump heating) and SEER (for air conditioning cooling), do a better job of estimating efficiency throughout an entire heating or cooling season. They consider the system’s performance across different temperatures and where it’s operating throughout the average year.

SCOP is a newer method of rating heat pumps designed to be a more realistic predictor than single point COP testing of performance in the real world. SCOP is defined as the total heat output over the season divided by the total input energy (kWh) over the same period.

SCOP values tend to be significantly higher for systems using more constant/consistent heat sources (eg, geothermal, seawater) than for air source heat pumps that are required to operate in the face of wild temperature swings.

Why do these matter? That’s because a system could have a perfectly respectable COP “at the test condition, during which they performed well,” but if its performance tanks in the extreme heat or cold that prevails where you live, then its seasonal efficiency (and your bill) will also tank.” Seasonal ratings are an attempt to capture that.

Finding Your System’s COP: The Real Cheat Sheet (Sort Of)

All right, now back to that scavenger hunt for your system’s COP. We already mentioned that product spec sheets are your friend. That’s the easiest way, ask your contractor.

But what if you cannot get one? Perhaps you’ve already got a system, and when you asked your contractor for spec sheets from 2008, she shot you the side-eye.

Here’s the lowdown and what to do, though they’re less than ideal:

For Cooling COP (Heat Pump or AC): You can calculate the coefficient of performance for cooling (COP) if you have the EER of the system (which is listed on directories such AHRI or CEE) since COP = EER x 0.293. This COP is calculated using EER test conditions (95°F outdoor temperature and 80°F indoor temperature).

For Heating COP (Heat Pump): This is a little tricker.

• Searching Online: You may find someone who posted spec data for exactly your system model combo. Hit or miss.
• Let’s Pretend Heating COP > Cooling COP: We’ll just assume here that a heat pump’s heating COP is going to be better than its cooling COP because you get the extra benefit of the compressor’s waste heat in heating mode. Not a number that I consider to be truthful, but one that’s probably not far off if you compare performance.
• Conversion to HSPF (Ballpark estimates): You can kind of sort of change HSPF into a Seasonal COP by multiplying by 0.293. BUT, please bear in mind, HSPF is an AVERAGE – conditioned on septempber and average specific regional climate assumption, and test cycles. It’s not the COP at any given slight temperature, and it might not be representative for your own climate.
• Capacity Based: If you are able to take capacity (at 47°F and 17°F) you may assume system electrical input @ 17°F and @ 47°F is about equal to the input it used during the EER test (you calculated it from EER/Cooling COP…) and estimate heating COP (Watts Out / Estimated Watts In). This is not strictly true; you do see a difference in input at different temperature differences, but gives a rough idea.

Why This Matters for Cool Climate Heating

It’s important to look at the heating COP and the capacity at that low temperature, (that 17°F number, for example) if you live somewhere that can get really cold.

• COP and heating capacity of heat pumps decrease with decreasing temperature.
• If the heat pump can’t cope, it depends on backup heating. This is typically electric resistance heat strips, with a COP of 1.0 (it takes 1.0 unit of electricity to get 1.0 unit of heat).
• Backup heat operation wastes electricity like mad compared to the heat pump.

Therefore, even if a heat pump has a fantastic COP at 47°F, if its COP and ramping capacity are greatly reduced at 17°F (or lower – data might not even be available for low operating temperatures!), you’ll be paying more for that inefficient back-up heat more of the time. Knowing the COP curve will help you determine if a heat pump is suitable for you given your particular climate or if you need a different primary heat source (such as natural gas).

COP, particularly when you consider how it varies with temperature, gives you a good idea of how the system is going to behave and how much energy it is going to consume, in your specific circumstances.

Final Thoughts on COP

So, what have we landed on? What is coefficient of performance? It’s a neat way to think about how heat pumps, air conditioners, and refrigerators work efficiently. It’s a guidance of how much useful output (heating/cooling) divided by the energy input. That’s why these systems can achieve performance figures (COP) greater than 1 – they are shifting heat, not generating it.

COP is a function of the operating conditions, mainly the temperature lift (difference in temperature between heat source and heat sink). The greater the T gap, the lower the COP. In addition, design of the system contributes to achieving maximum COP.

And while instantaneous COP is what you get at a moment in time, seasonal measures like SCOP, SEER and HSPF show you how a system will perform over a full year.

The COP rating specifications for residential equipment, particularly heating COP for heat pumps, are difficult to find on the label (although not in all cases) but are readily available on the product’s spec sheet. Cooling COP can often be approximated from EER.

To know COP is to have a secret decoder ring to system performance. It’s a way to see past glossy brochures and determine whether a system really is a good match for your climate and your home, which has its own heat loss/gain characteristics. It is merely a part of the puzzle, but a hell of an important one. Coupled with other pieces of information about your home and local weather, COP is what you use to make smart decisions and possibly save some money on those bills. Admittedly, that is the true victory with what is coefficient of performance.

COP FAQs

Alright, quick hits on some common questions about COP.

Q: What is the meaning of Coefficient of Performance (COP)? A: It’s a rating that quantifies the efficiency of moving heat for such devices as heat pumps, refrigerators or air conditioners. It’s the ratio of the useful heating or cooling you get, to the energy input the system needs in order to do the work.

Q: How are COP and efficiency different? A: Efficiency, such as thermal-efficiency, typically involves transferring form of energy into another (such as a fuel into heat) and is described as a ratio where 100% efficient (a ratio of 1) being as close to achieving the theoretical maximum (total conversion) as possible. However, COP is for devices that move heat, not generate it. Since it takes more work to create than to move the heat, the output to input ratio (COP) can be and often is greater than one. It is a “performance ratio,” not exactly an “efficiency ratio” as you put it.

Q: What are the dimensions of COP? A: COP is an output energy/power/input energy/power so they cancel. It’s a dimensionless number.

Q: Is a higher or lower COP number better? A: Higher is better. A higher efficiency — COP is a measure of efficiency — means the air-source system is delivering more heating output or cooling power for each unit of energy input. This is reflective of greater efficiencies and reduced costs of operations.

Q: How does this affect the COEFFICIENT of performance as a function of temperature? A: Cop is heavily influenced by temperature. Normally, the greater the temperature gradient the system must overcome (the distance between the hot and low-temperature reservoirs), the smaller the COP. As the temperature of the heat source increases (heating) or heat sink decreases (cooling), the COP tends to rise.

Q: Can COP be greater than 1? A: Absolutely, yes. This is an important characteristic of heat pumps, refrigerators, and air conditioners. Their COP, greater than 1, results from just moving heat from one locati0n to another, so above 1 means that they aren’t turning an energy input into heat. You use less energy moving heat than the energy you use generating heat from nowhere.

Q: What are SCOP and SEER? A: The SEER (Seasonal Energy Efficiency Ratio) and SCOP (Seasonal Coefficient of Performance) are analogous, and are designed to give a real world measure of how efficient a system is over a full year/season of heating/cooling. In contrast to the single point, instantaneous COP, these seasonal COPs reflect performance levels averaged over the temperature and operating conditions normally experienced over the course of a year.

Q: How can heating COP for the same heat pump be higher than cooling COP? A heat pump produces heat similarly to an a/c, but on purpose…. When a service heats there is an input of electrical energy, energy going to the driving the compressor and the fans, that is converted into heat. This heat “waste” from the components compounds the heat being absorbed from the outside and fed into the house. Well, the total heat output is the heat moved plus the work input, so you get (Heat Moved + Work Input) / Work Input – higher than the heating COP (Heat Moved) / Work Input by a factor of 1, assuming the same benchtop. For cooling operation, this heat is an undesirable component and must be removed as well.