COP or Coefficient Of Performance is the most basic energy-efficiency metric of any heat engine. It’s instrumental when comparing heat pumps, fridges, and air conditioners.
Definition: COP is the ratio of how much useful heat (or cold) a heat pump will produce if we give it certain energy input.
Basically, it tells us how much heat we can generate with every watt of energy.
Do keep in mind that the amount of electricity used is the same in both cases. Running a 1000W heater for an hour costs about 13 cents.
On average, a COP 3.5 heater will boil a gallon of water for less than 1 cent. COP 2 heater will boil a gallon of water for more than 1 cent.
It’s much better to have a COP 3.5 water heater than a COP 2 water heater. For example, the best tankless water heaters are known to have a high coefficient of efficiency.
Example: Even a small electric tankless water heater is powered by 9,000W. Bigger ones with 15+ GPM can consume as much as 36 kWh every hour. The high COP of such a heat pump is essential for electricity cost-optimization. On the other hand, devices that don’t spend a lot of energy – something like battery-powered portable air conditioners – have a low COP.
Let’s look at how COP is calculated, what is the highest possible COP of a heat pump, and how the electricity bill is affected by HVAC units with different COP values.
How To Calculate COP? COP Formula
Here is the COP formula (an equation that calculates the coefficient of performance for any heat pump):
COP = Q/W
where Q is the heat the heater generates if we give it a certain amount of work (W).
For cooling, Q represents the heat that we take away from a cold reservoir. An air conditioner, for example, takes heat away from a room (cold reservoir).
Note: COP for heating and COP for cooling may be different. The best mini-split heat pumps, for example, are capable of cooling down space as well as heating it.
A good mini-split system will usually have a cooling COP ratio of 2 or more, and a heating COP ratio of 3 or more.
If we apply the 2nd law of thermodynamics and do a bit of derivation, we can see the COP values for a theoretically 100% ideal heat pump and ideal air conditioner (we also call that a Carnot machine). Let’s do the heat pump first:
COP Of A Heat Pump
Here’s how you can calculate the theoretically maximum COP of a heat pump:
COPheat pump = Thot/(Thot-Tcold)
Thot is that cozy hot temperature we want to have during cold winters (let’s say 95F; that’s 308 in Kelvins).
Tcold is the cold temperature at which the heat pump starts to operate (57F or 287K).
Theoretical maximum COP is calculated like this:
COPheat pump = 308K/(308K-287K) = 14,67
So, in theory, a heat pump can have a COP even above 10. Nonetheless, the real COP of a heat pump in practice is a lot lower.
Golden standard: The standard test to measure the COP of a heat pump is conducted with Thot = 95F (308K) and Tcold = 32F (273K). That means that in a 100% ideal case the maximum COP is 8.8. But in practice, it’s lower.
In fact, the highest COP a heat pump can achieve is about 4.5. Any heat pump with a COP of above 3 has very high energy efficiency. Ductless mini split heat pumps readily use this efficiency; you can read more about what is a mini split here and what are disadvantages of mini splits at very low temperatures here (COP drops below 1.0).
Here is a graph of how much more efficient high COP heat pumps are. We put the COP 2 as zero and calculated by how many percent more efficient the higher COP heat pumps are.
You can see, for example, that a 3.2 COP pump is 60% more energy-efficient than 2 COP pumps.
Coefficient Of Efficiency Of Air Conditioners
OK, let’s calculate the maximal theoretical efficiency of a cooling device. Namely, an air conditioner or a refrigerator. By applying the 1st law of thermodynamics, we can deduce the ‘Carnot COP’ for a cooling device is calculated as such:
Tcold is the chill temperature you want to have in your room during the summer. Thot is the high heat wave temperature.
Let’s look at what would a COP of an air conditioner be in a standardized Thot = 95F (308K) and Tcold = 32F (273K) temperature interval. Plugging the temperature in the COP cooling equation above, we get 7.8.
If you remember, the maximum coefficient of performance of a heat pump was 8.8. If you are in the market for an air conditioner, make sure to get one with a COP value of above 2. That’s a very high COP for a practical HVAC cooling device.
Usually, the problem is that you won’t find a COP value anywhere, not even on a specification sheet. Energy-efficiency is usually represented with such metrics as EER and SEER; all these as based on the COP of an HVAC divide. For example, we have compared the best portable air conditioners by comparing their EER ratings.
You can also check how heat pumps work in general and see which heat pumps have the highest SEER rating here.
Seasonal COP Or SCOP
In 2013, SCOP or Seasonal Coefficient Of Performance was introduced. We know that COP is a measure of energy efficiency for a heating or cooling device. What measuring SCOP is trying to achieve is to objectively measure energy efficiency over the winter season (for heating) and summer season (for cooling).
Basically, the relationship between SCOP and COP is the same as with SEER and EER.
SCOP would give a much more realistic idea about how energy efficient an HVAC device is in practice, i.e., in a real summer season.
Nonetheless, the SCOP has still considered a very new methodology of measuring seasonal cooling and heating efficiency. As such, you will rarely find the SCOP ratio on older devices. In fact, even the new devices rarely include SCOP in their specification sheets, primarily because they have yet to measure it.