Heat Pump COP at 15 C - Efficiency by Pump Type

What COP tells you about your heat pump

COP stands for Coefficient of Performance. A COP of 3.5 means the heat pump delivers 3.5 kWh of heat for every 1 kWh of electricity it consumes. The extra 2.5 kWh comes free from the outdoor air, ground, or water source. This calculator works in two modes: enter the heating output and electrical input from the spec sheet for an exact COP, or enter source and supply temperatures for a Carnot-based estimate. Add your electricity rate and the calculator shows the cost per kWh of heat and compares it against gas, oil, propane, and electric baseboard heating.

How to use this calculator - step by step

Calculation method - choose "From power data" if you have the manufacturer spec sheet with heating output and electrical input in kW. Choose "From temperatures" if you want to estimate COP based on your local conditions.
Heat pump type - select air-source (most common), ground-source (geothermal), or water-source. This affects the Carnot efficiency factor used in temperature-based calculations.
Power data or temperatures - enter either output/input (kW) or source/supply temperatures (C). The calculator handles the rest.
Electricity rate (optional) - enter your rate in $/kWh to see cost per kWh of heat and a comparison against other heating fuels.
Read results - COP value with efficiency class, cost per kWh, heating cost comparison table, and the COP efficiency scale showing where your pump falls.

COP by heat pump type and outdoor temperature

Estimated COP values at 35 C supply temperature (underfloor heating) based on Carnot model with real-world efficiency factors.

Source temp (C)	Air-source	Ground-source	Water-source	Carnot max
-15	2.59	2.96	3.08	6.16
-10	2.88	3.29	3.42	6.85
-5	3.24	3.70	3.85	7.71
0	3.69	4.22	4.40	8.80
5	4.31	4.93	5.14	10.27
10	5.18	5.92	6.16	12.33
15	6.47	7.40	7.71	15.41

Real-world examples

Air-source in mild winter (7 C)
12 kW output, 2.5 kW input, $0.16/kWh
COP: 4.80 - cost per kWh heat: $0.033

Air-source in deep freeze (-15 C)
8 kW output, 3.5 kW input, $0.16/kWh
COP: 2.29 - cost per kWh heat: $0.070

Ground-source, underfloor (10 C source, 35 C supply)
Temperature method, $0.14/kWh
COP: 5.92 - cost per kWh heat: $0.024

Air-source with radiators (-5 C source, 55 C supply)
Temperature method, high supply temp, $0.18/kWh
COP: 2.30 - cost per kWh heat: $0.078

Water-source, well water (12 C source, 40 C supply)
Temperature method, $0.12/kWh
COP: 5.59 - cost per kWh heat: $0.021

Frequently asked questions

What is the difference between COP and SCOP?

COP is measured at a single operating point (e.g., A7/W35 means 7 C air, 35 C water). SCOP (Seasonal COP) is the weighted average COP across an entire heating season, accounting for varying outdoor temperatures. SCOP is a more realistic measure of annual performance. A unit with COP 4.5 at A7/W35 might have an SCOP of 3.2-3.8 depending on your climate. EU energy labels use SCOP for efficiency classes.

Does COP drop significantly in cold weather?

Yes. Air-source heat pumps lose about 30-40% of their COP when outdoor temperature drops from 7 C to -15 C. A unit rated COP 4.5 at 7 C might deliver only COP 2.5 at -15 C. Ground-source pumps are much more stable because soil temperature stays at 8-12 C year-round. This is why SCOP matters more than a single COP number for cold climates.

Is a heat pump cheaper than a gas furnace?

At US average electricity ($0.16/kWh) and gas ($1.20/therm) prices, a heat pump with COP 3.5 costs about $0.046/kWh of heat versus gas at $0.041/kWh (at 92% efficiency). Nearly equal. At COP 4.0+, the heat pump wins. In states with cheap electricity or expensive gas, heat pumps are clearly cheaper. In states with very cheap gas (Texas, Oklahoma), gas furnaces can still be more economical for heating only - but heat pumps also provide cooling.

What supply temperature should I aim for?

Lower supply temperature = higher COP. Underfloor heating at 30-35 C gives the best performance (COP 4-5+). Fan coils work well at 35-45 C. Modern low-temperature radiators need 45-55 C. Old high-temperature radiators at 60-70 C severely hurt COP (often below 2.5). If you have old radiators, consider replacing them or adding underfloor heating in the main living areas before installing a heat pump.

What is the Carnot COP and why is actual COP lower?

The Carnot COP is the theoretical maximum efficiency for a heat pump operating between two temperatures: COP_carnot = T_hot / (T_hot - T_cold), where temperatures are in Kelvin. Real heat pumps achieve 40-50% of the Carnot limit due to compressor losses, heat exchanger inefficiency, fan power, defrost cycles, and refrigerant properties. Air-source pumps typically reach 42% of Carnot, ground-source 48%, and water-source 50%.

Can a heat pump work at -20 C or colder?

Modern cold-climate air-source heat pumps (models with enhanced vapor injection or EVI compressors) can operate down to -25 C to -30 C. COP at those temperatures drops to 1.5-2.0, but the unit still provides heat. Many installations include a backup electric resistance heater that kicks in when COP falls below a useful threshold. Ground-source pumps avoid this problem entirely since the ground stays warm.

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Heat pump COP at 15 degrees - how efficient is your system?

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