Heat pump efficiency depends on outdoor temperatures. As we all know, at lower temperatures, heat pumps become less efficient. That’s why we usually use a furnace combo with heat pumps; at freezing temperatures, heat pumps become less effective, and we have to use the furnace for heating.
How much does heat pump efficiency drops in cold climates is described by the heat pump efficiency vs temperature graph (for temperatures between 0°F To 60°F).
Below you will find both a heat pump efficiency vs temperature chart and a table with heat pump COP values at low temperatures. We will also look at an example of how 24,000 BTU heat pump output falls below 15,000 BTU at low temperatures at the end.
Before we check the heat pump temperature efficiency chart and the table, let’s address the key metric we are using:
Heat pump efficiency is measured by COP or Coefficient Of Performance. Heat pump COP is defined as:
Heat Pump COP = Heating Output / Energy Input
Basically, when we are measuring heat pump efficiency, we are measuring COP. At higher temperatures (about 52°F and above), the heat pump coefficient of efficiency can be above 4. That means that a heat pump will produce 4 times as much heating output for every 1 unit f energy output. In short, a heat pump will have 400% efficiency.
All heat pumps have above 100% efficiency because they do not generate heat from electricity. They merely use electricity to pump available outdoor heat indoors.
Obviously, the efficiency of heat pumps depends on how much heat is available in outdoor air. That simply means that we have a relationship between the outdoor temperature and heat pump efficiency. In colder climates, a heat pump can extract less heat from the cold outdoor air. COP at low temperatures completely depends on the capability of the outdoor unit (indoor unit doesn’t influence the overall efficiency).
Example: An average heat pump efficiency at 45°F is about 3.7 COP. That is 370% efficiency. At much lower temperatures – say 10°F winter temperatures – an average heat pump efficiency is about 2.3 COP. That is 230% efficiency. Relatively speaking, we will see that a heat pump at 10°F will produce 38% less heat than at a higher 45°F temperature.
Let’s say that we have a 24,000 BTU heat pump. This heat capacity is usually measured at 47°F; that means that at 45°F such a heat pump will actually produce about 24,000 BTU of heating output (or a little below that). At 10°F, however, we will see a 38% decrease in heating output; a 24,000 BTU heat pump will thus produce only 14,880 BTU or even less.
How much COP drops at lower temperatures is a matter of measurement (measuring heating output for certain electricity input at various temperatures). A limited number of studies were conducted for low single-digit temperature heat pump efficiencies.
The most all-encompassing study on heat pump efficiency at lower temperatures was conducted by the US Department of Energy. The “Measured Performance of a Low Temperature Air Source Heat Pump” study was conducted in September 2013. Based on the results from that study, we can roughly chart heat pump efficiency vs temperature at low temperatures:
Heat Pump Efficiency Vs Outside Temperature Chart
This heat pump COP vs temperature chart will help you understand the efficiency of heat pumps at low temperatures. Here is the full heat pump output temperature chart, based on the measured heating outputs from the DOE study:
As we can see from the heat pump temperature efficiency chart, we are seeing very high COP 4 efficiency at higher temperatures (above 52°F. At below 32°F, we see the heat pump efficiency falling below COP 3 (300% efficiency).
At single-digit temperatures, the efficiency of heat pumps drops by almost half compared to 47°F temperature (HSPF rating, for example, is measured at 47°F). This doesn’t mean that heat pumps stop working at freezing temperatures; it just means they are less efficient and thus produce lower heating output than the one we have on the label.
Note: These are low-temperature heat pumps. The COP value of standard heat pumps will fall much quicker (with a COP rating of about 1.5 at 10°F).
Here is the promised chart of COP vs temperature for heat pumps:
Heat Pump COP Vs Temperature Table
| Outside Temperature (°F): | Coefficient Of Performance (COP): |
| 60°F | 4.3 COP |
| 55°F | 4.1 COP |
| 50°F | 3.9 COP |
| 45°F | 3.7 COP |
| 40°F | 3.3 COP |
| 35°F | 3.1 COP |
| 30°F | 2.9 COP |
| 25°F | 2.6 COP |
| 20°F | 2.5 COP |
| 15°F | 2.4 COP |
| 10°F | 2.3 COP |
| 5°F | 2.25 COP |
| 0°F | 2.2 COP |
It needs to be noted that there is currently a challenge to create heat pumps that can heat efficiently at near-zero temperatures as well. One interesting company has already managed to increase COP ratings at very low (5°F) temperatures.
The new Cooper and Hunter Hyper Heat series gives us insight into what efficiency modern very low-temperature heat pumps are capable of. If you check the specification sheet of the Hyper Heat series here, you will see COP ratings for 5°F temperatures. Namely, you can see that:
- The small 7000 BTU HPR Series CH-HPR06F9-230VO heat pump has a 2.2 COP rating at 5°F temperatures.
- Bigger 24000 BTU HPR CH-HPR24-230VO heat pump has a 1.7 COP rating at 5°F temperatures.
What does this lower COP at low temperature actually mean in terms of heating output?
Let’s make some theoretical calculations and look at the heat pump output of a 24,000 BTU heat pump at different temperatures:
Heating Output Of 24,000 BTU Heat Pump At Different Temperatures (Example)
Let’s say that we have a 24,000 BTU (2-ton) heat pump that has about 24,000 BTU of heating output at 45°F at 3.7 COP. Based on the COP vs temperature study results above, we can calculate how many BTU will this heat pump produce at lower temperatures.
Here are the rough 24,000 BTU heat pump BTU output estimates at various temperatures:
- At 45°F, this heat pump has 3.7 COP and will thus produce 24,000 BTU of heating output.
- At 40°F, this heat pump has 3.3 COP and will thus produce 21,405 BTU of heating output.
- At 35°F, this heat pump has 3.1 COP and will thus produce 20,108 BTU of heating output.
- At 30°F, this heat pump has 2.9 COP and will thus produce 18,810 BTU of heating output.
- At 25°F, this heat pump has 2.6 COP and will thus produce 16,865 BTU of heating output.
- At 20°F, this heat pump has 2.5 COP and will thus produce 16,216 BTU of heating output.
- At 15°F, this heat pump has 2.4 COP and will thus produce 15,568 BTU of heating output.
- At 10°F, this heat pump has 2.3 COP and will thus produce 14,919 BTU of heating output.
- At 5°F, this heat pump has 2.25 COP and will thus produce 14,596 BTU of heating output.
- At 0°F, this heat pump has 2.2 COP and will thus produce 14,270 BTU of heating output.
As we can see, even with a low-temperature heat pump, the efficiency can fall so much that we lose almost half the capacity of a heat pump. That plays a major role when we have to size a heat pump. You can read more about how many BTU mini split heat pump you need here; you will see that none of the estimation tools take the drop of COP at lower temperatures into account.
All in all, it is important to understand the relationship between heat pump efficiency and outdoor temperature. In recent years, a lot of resources have been dedicated to inventing a heat pump with very good COP at low temperatures.
Essentially, because heat pumps are fuelled by electricity, they are considered ‘green’. If we can get the COP values at low temperatures us, they could be a financially viable and environmentally friendly alternative to gas furnaces.
We hope you now have an idea of how heat pump efficiency and outdoor temperatures are connected. For more in-depth and scientific studies, you can check Engineering Toolbox here.
Your COP vs Temperature graph is only useable when an accompanying SEER value for the heat pump is given. Do you have a graph that provides this?
Hi Dave, SEER is a measure of energy efficiency in cooling mode. In the winter (low temperatures), we use heating mode and the accompanying efficiency rating is the HSPF rating, not the SEER rating. However, HSPF is measured at specific test conditions; that’s why COP is pretty much the only relevant metric that can be used here.
So yes…I should have requested a graph of COP vs temperature for a Trane model being marketed with a SEER rating of 18. Do you have one?
Hi Dave, these COP vs temperature graphs for specific models are not available. Maybe Trane has them, you might try to reach out to them to see if they can provide it.
I’m located near Toronto Canada. I have a new 2 ton heat pump with new high efficiency forced air 60k btu gas furnace as a backup for cold days. I’m wondering about the economic balance point for my system, the temperature at which one fuel is more economic than the other to use. I set my heat pump to operate down to 40 F and then switch to the high efficiency gas furnace at colder temp, under the impression that the furnace is cheaper to run (cost of natural gas plus electricity for the furnace fan(2-3 amps)) than the heat pump (cost of electricity for heat pump and furnace fan) once temperatures get near or below freezing. The heat pump has HSPF 11.6, COP 3.8 at 47F and 2.6 at 17F. I’m wondering if this is the most economical setup. Or is there room to run the heat pump to a lower temp without getting into high cost for electricity. In the winter we set back heating temp a few degrees while we are at work and sleep so the heating system has a break during the day time when electric costs the most.
Hi Jeff, thank you for a very interesting question and especially about the temp-dependent COP values. This is an age-old question; at which temperature should you switch from heat pump to furnace? You will see a lot of temperatures being thrown around (from 32 degrees F, to 35F, even 40F, and so on). The real answer is this; it depends a lot on both heat pump and furnace efficiencies and, above all, cost of electricity vs natural gas.
Here is how to think about this: At 47F, the COP is 3.8. That means that you get 3.8 kWh worth of heating for every 1 kWh of electric input. At 17K, the COP is 2.6 and you get 2.6 kWh worth of heating for every 1 kWh of electric input. In most cases (depends on gas vs electricity prices), it makes sense to heat with a heat pump at 47K. At 17K, for example, the heating output of your heat pump decreases by 32% compared to 47F efficiency.
That is not all that much; if the electricity is cheap in your area and gas is a bit expensive, it might be that it would actually be more economical to run a heat pump even at 17F outdoor temperature.
Given that your heat pump still have a 2.6 COP at 17F, the effect on your electricity bill might be 30%, 40% higher than getting the same heat output as at 47K. The price of electricity and natural gas plays the most important role here; thus it is almost impossible to accurately calculate the exact temperature when you should switch from heat pump to gas furnace.
The best advice in this situation would be a very rough estimation; when the temperatures hit 32 degrees, switch to the gas furnace. The thing is that 2 ton heat pump generates 24k BTU heat output; your furnace can do 60k BTU. At lower temperatures, you need more heating, and with decreasing temperature, the gas furnace will become an ever more viable option.
I’m sorry the recommendation cannot be more exact. The Canadian government is currently incentivizing high COP low-temp heat pumps; it’s a program with a great goal, we will look closely how the newer heat pumps can be increasingly more efficient even in cold climates.
Cost Comparison of 1 kWh of Heat in Ontario March 2023
m3 kWh efficiency price/heat in $CDN
gas/m3 0.323821 0.031 0.8 0.038
electricity/kwh off peak 0.074 1 0.074
electricity/kwh mid peak 0.102 1 0.102
electricity/kwh on peak 0.150 1 0.150
Electricity leveraged with heat pump:
outdoor F outdoor C pump COP peak mid peak off peak
11am-5pm 7pm-7am
60 15.6 4.3 0.035 0.024 0.017
55 12.7 4.1 0.037 0.025 0.018
50 10.0 3.9 0.038 0.026 0.019
45 7.2 3.7 0.041 0.028 0.020
40 4.4 3.3 0.045 0.031 0.022
35 1.6 3.1 0.048 0.033 0.024
30 -1.1 2.9 0.052 0.035 0.026
25 -3.9 2.6 0.058 0.039 0.028
20 -6.7 2.5 0.060 0.041 0.030
15 -9.4 2.4 0.063 0.043 0.031
10 -12.2 2.3 0.065 0.044 0.032
5 -15.0 2.25 0.067 0.045 0.033
0 -17.8 2.2 0.068 0.046 0.034
Conclusion: It has more to do with the time of day than anything. Off peak heat pumps are almost always cheaper than gas. On peak they almost never are. So get a wood stove, a chain saw and a splitter, lol.
Hi Leif, that’s quite an extensive set of information that you have put a lot of effort into writing them down here. Thank you so much for this real-world example. As you have correctly concluded, the price of electricity (off-peak vs on-peak) is of paramount importance here.
You’re probably better off switching to tiered pricing in the winter. You want to be heating when it’s warmest outside as much as possible (late afternoon/early evening), and that’s when Time-of-Use rates are highest.
Here’s an update of the table above with the tiered pricing rates:
Electricity leveraged with heat pump:
outdoor F – outdoor C – pump – COP – first 1000kWh’s – Additional kWh’s
60 15.6 4.3 0.020 0.024
55 12.7 4.1 0.021 0.025
50 10.0 3.9 0.022 0.027
45 7.2 3.7 0.024 0.029
40 4.4 3.3 0.026 0.031
35 1.6 3.1 0.028 0.033
30 -1.1 2.9 0.031 0.036
25 -3.9 2.6 0.033 0.040
20 -6.7 2.5 0.035 0.042
15 -9.4 2.4 0.036 0.043
10 -12.2 2.3 0.038 0.044
5 -15.0 2.25 0.039 0.046
0 -17.8 2.2 0.040 0.047
We run ours to ~-12C here in Ottawa, with the thermostat set to between 17C and 21C depending on the time of day.
The electrical input should be about constant, the motor-compressor running, though I imagine it could vary some with varying temperature related freon pressures (I recall reading that the heat available at 60F ambient is nearly overwhelming to the heat pumping requirement…for a single speed motor-compressor). So, as long as your system is running it is pumping less heat as ambient temperature falls…at a time when you need more heat. Back-up heaters are necessary. In south Texas, our cheapo systems would not be specially designed for very low temperatures, so likely running at 1.5 COP at 5F (per the article). Our backup heat is electric resistance coils in the inside air handler…delivering heat at almost 1:1…so our “four ton” unit (at 47F) at 5F is putting out 1.7 ton of heat from the heat pump and possibly 2.3 ton of heat from resistance coils (“4 tons”). Oh my! The meter is spinning crazy! And the house is only 60F inside. With all the economic growth in Texas the suburban sprawl of housing, “all electric,” is enormous…I fear our ERCOT regulators and our market system just ignores this low temperature problem because it is an “anomaly” to a rational cheapest possible cost…lowest price planning (but, the long tail of very low probabilities bights hard when it hits). The “anomaly” became reality as extremely low temps fell all across Texas in February 2021 and the power supply failed for a large part of the populace for several days. No rolling blackouts, once off…staying off. There were many cold and snowy weather-related difficulties…but even so, I doubt that Texas has enough operable power generating capacity to meet such an extreme winter load.
Here is my own experience from our view overlooking modern tract housing, about two hundred feet below us in a valley. These homes are all electric, typically 1,400 to 2,000 square feet, all wrapped in TYVEX during construction, energy efficient, sitting on small lots. For a clear sky night, the “point forecast low” is 30F…our gage and my truck both say it is 33F…driving to work as I pass through the subdivisions my truck says 16F and everything is covered in sunlit sparkling frost. As those homes lose heat, the warmed air rises up the hillside, to our benefit. As these homes sit low in the valley, the cold air exhausted out of their heat pumps has no where to go.
On clear nights, valleys have always, (w/o human influence) been colder than than the hill tops. Surfaces are cooled below ambient by radiative heat loss to the upper atmosphere and the air cooled by contact with these surfaces flows downhill.
can you suck warmer air out form under the house into a heat pump with a flip up top , build a inclosed sides and top and get more heat on cold winter nights? Air under my house is about 59 degrees . Thinking about putting a fan with vent doors that open when heat pump comes on blowing the heat from underneath house around heat pump to get the ouside air up to a higher temp. Do you think this would work ?
Hi Joel, this is a great idea; you can definitely do that. We had a case where the outdoor unit was installed in a garage in a 100-flat building. While the outside temperature was extremely low, the temperature in garage was quite high. The efficiency there was superb even at below zero temperatures. Go for it.
I live in Vermont and may soon be confronted with converting to a heat pump. I currently use heating oil. I used info from this great article to calculate my cost each month using a heat pump and compared to my cost for heating oil. Problem is that the result is that a heat pump is much more costly to operate than heating oil. That seems contrary to all the acclaim about heat pumps being extremely efficient and proponents for legislation to eliminate fossils claiming heat pumps will save money. Are my calculations flawed?
Electrical Cost Using a Heat Pump – 24,000 BTU; 20 AMP
Criteria January
Gallons of Fuel Oil Used 97
BTUs Consumed 140,000 13,580,000
Efficiency Loss – Fuel Oil 15% (2,037,000)
BTUs Required – Heat Pump 11,543,000
Average Temp 20
BTU Output Based On Temp 16,216
Hours of Run Time 711.83
KWH / Hr of Run Time 4.40 3,132.04
Cost / KWH $0.1928 603.86
Cost – Fuel Oil $4.0000 388.00
Heat Pump – Better / (Worse) (215.86)
Hope my columns stay in place. Appreciate any help or advice you can give me.
Hi Joe, alright, we’re going to do the math here, but let’s first point out: Heat pumps are very efficient (300%+ efficiency), but electricity is expensive. When we talk about cost-efficiency, heating oil is sometimes still a cheap choice still (despite your furnace’s 85% efficiency). Comes down to your electricity rates and heating oil prices, basically.
Now, the math: You have correctly calculated that 11,543,000 BTU heating output requirement. Good job there. Now, the key thing here missing is the HSPF rating of the heat pump. A good heat pump will have a 12 HSPF rating, which means that it runs on 24,000 BTU / 12 HSPF = 2,000 watts. Now, if you have calculated that the BTU at 20 degrees is not 24,000 BTU but 16,216 BTU (seems right, can you explain the calculation for everybody else here?), that means that you pay for 2 kWh for 16,216 BTU.
You have also correctly calculated that you would need 711.83 running hours to match the furnace heating output. We know that 1 running hour uses 2 kWh of electricity; that means that 711.83 running hours will use 1423.66 kWh.
Multiplying this by your electricity rate ($0.1928/kWh), we get the total running cost: $274.48. Compared to $388.00 for heating oil, heating with heat pump is saves you more than $100 per winter.
Looking at your calculations, I think the error might be that the presumption was that a heat pump is 100% efficient. Space heaters, for example, are 100% efficient but they basically “burn” electricity to produce heating output. Heat pumps just pump the heat from outside into indoors; that’s why they have a 300%+ efficiency. Because you are looking to use the heat pump at 20 degrees, the efficiency will fall closer to 200%.
Now, at 100% efficiency, you get to the $603.28. With a bit more than 200% efficiency, you get to $274.48.
I hope this makes sense and, I have to point out, spendid maths on your part.
I understand the difference now. Using the HSPF rating each running hour generates 2 KWH. I calculated 4.4 KWH (based on the unit using 220 volts at 20 amps). When I use 2 KWH, I come right back to your $274.48. Guess I need to read up on HSPF ratings.
16,216 BTU (seems right, can you explain the calculation for everybody else here?) – I’d like to take credit, but I got it from the table included in your article above. Since then, though, I’ve looked other heat pumps and included in the specifications are generally the COP and BTUH for a high and low temperature from which a range of btu outputs, temps, and cop can be developed linearly. May not be exactly right but good enough for my purposes.
By the way, these numbers are for one month (January). Saving $110 for January and $550 for a year will help the unit pay for itself over time although there would be some cost for deicing.
Thanks a lot. You nailed it.