Condenser Heat Rejection Calculator
A condenser has to get rid of more heat than the evaporator picks up: everything absorbed indoors plus the energy the compressor puts in. Enter the capacity and the compressor input to see the total.
Method reviewed October 26, 2026. Version 1.0.0. How it works
Result
- Total heat of rejection
- 46,236BTU/h
- Heat-rejection ratio (rejected ÷ absorbed)
- 1.284
The condenser rejects about 46,236 BTU/h — 1.28 × the cooling capacity.
- This assumes all of the compressor’s electrical input ends up as heat in the refrigerant, as it does in a hermetic or semi-hermetic compressor cooled by suction gas. For an open-drive compressor, use the shaft power instead.
- Compressor input as heat
- 10,236BTU/h
- Compressor input
- 3kW
- Cooling COP
- 3.52
Show the working
- Compressor input as heat = kW × 3,412.143 × 3,412.14 = 10,236 BTU/h
- Energy balance: heat rejected = heat absorbed + work in36,000 + 10,236 = 46,236 BTU/h
Why the condenser rejects more than the cooling capacity
Energy is conserved around the refrigeration cycle. Over each cycle the work put into the compressor equals the heat sent to the hot side minus the heat taken from the cold side. Rearranged, the heat rejected at the condenser is the heat absorbed in the evaporator plus the compressor input.
In a hermetic or semi-hermetic compressor, the motor is cooled by the suction gas, so practically all of its electrical input ends up in the refrigerant. That is why this calculator adds the electrical input, converted at 3,412 BTU/h per kW.
The heat-rejection ratio
Dividing the heat rejected by the cooling capacity gives a ratio of 1 + 1/COP. A system with a COP of 4 rejects 1.25 times its cooling capacity; a less efficient system, or the same system on a hotter day, rejects more.
- Use capacity and power at the same operating conditions.
- EER is BTU/h per watt, so input watts = capacity ÷ EER.
- COP is the same ratio in consistent units: COP = EER ÷ 3.412.
Worked example
These are the example values the calculator opens with, and the result the engine calculates for them.
Inputs
- Cooling capacity (heat absorbed in the evaporator)
- 36,000 BTU/h
- Compressor input known as
- Electrical power (kW)
- Compressor electrical input
- 3 kW
Result
- Total heat of rejection
- 46,236 BTU/h
- Heat-rejection ratio (rejected ÷ absorbed)
- 1.284
- Compressor input as heat
- 10,236 BTU/h
- Compressor input
- 3 kW
- Cooling COP
- 3.52
The condenser rejects about 46,236 BTU/h — 1.28 × the cooling capacity.
Assumptions
- Heat rejected = heat absorbed + compressor input (first law, steady state).
- All compressor input power becomes heat in the refrigerant stream.
Limitations
- Heat lost from the compressor shell and lines to the surrounding air is not separated out.
- Use capacity and power at the same operating conditions; both change with indoor and outdoor temperatures.
Frequently asked questions
How much heat does a 3-ton air conditioner reject?
With a 3 kW compressor input, about 46,200 BTU/h: 36,000 BTU/h of cooling plus about 10,200 BTU/h from the compressor.
Does the outdoor fan count?
No. The fan moves the air but its heat goes into the airstream, not the refrigerant. Enter only the compressor input.
What about open-drive compressors?
Their motor sits outside the refrigerant circuit, so only the shaft power reaches the refrigerant. Enter the shaft power in kW instead of the electrical input.
Sources
- University Physics Volume 2, §4.3 Refrigerators and Heat Pumps — OpenStax, Rice University (CC BY 4.0). Energy conservation W = Q_h − Q_c; refrigerator coefficient of performance K_R = Q_c ÷ W; heat pump K_P = Q_h ÷ W.
- NIST Guide to the SI (SP 811), Appendix B.9 — conversion factors — National Institute of Standards and Technology. 1 ton of refrigeration = 12,000 BTU/h; 1 BTU/h = 0.2930711 W; 1 atm = 101.325 kPa.
Spotted an error? Report a problem with this calculator — see our corrections policy.