What Does Horsepower Measure?
Horsepower (HP) is a unit of power. In older air conditioning terminology it was commonly associated with the size or power class of the compressor motor.
The compressor is only one component of an air conditioning system. The actual amount of cooling or heating produced also depends on the design and efficiency of the compressor, heat exchangers, fans, refrigerant circuit, electronic controls and the operating conditions.
Why kW Is a Better Measure
The purpose of an air conditioner is to move heat. Its useful output is therefore the amount of heat it can remove from a room in cooling mode, or add to a room in heating mode.
This is expressed as capacity in kilowatts (kW). Using kW allows different systems to be compared on the amount of cooling or heating they can actually provide, rather than simply comparing the nominal power of one component.
| Measurement | What It Describes | Useful for Air Conditioner Selection? |
|---|---|---|
| Horsepower (HP) | Power or a traditional compressor/motor size class | No – it does not directly state cooling or heating output |
| Cooling Capacity (kW) | How much heat the system can remove from the conditioned space | Yes |
| Heating Capacity (kW) | How much heat the system can deliver to the conditioned space | Yes |
| Electrical Input (kW) | The electrical power consumed by the system while operating | Useful for energy use – but it is not the same as capacity |
Capacity kW and Electrical Input kW Are Different
One point that can cause confusion is that both air conditioning capacity and electrical consumption can be expressed in kW, but they measure different things.
- Cooling or heating capacity (kW) is the useful thermal output of the air conditioner.
- Electrical input (kW) is the electrical energy the system is using at that moment.
A heat-pump air conditioner transfers heat rather than simply converting electricity directly into heat or cooling. For that reason, the cooling or heating capacity can be several times greater than the electrical input under rated operating conditions.
Inverter Air Conditioners Make Horsepower Even Less Useful
Modern inverter air conditioners continuously vary compressor speed to match the load in the room. The compressor may operate at a low speed when very little capacity is required and increase speed when more cooling or heating is needed.
Because compressor power is continually changing, a single horsepower figure is particularly poor at describing the actual performance of a modern inverter system.
Why Two “Same Horsepower” Systems May Perform Differently
Two air conditioners described using the same traditional horsepower class can have different actual cooling capacities, heating capacities and efficiencies. Differences can result from:
- Compressor design and efficiency
- Heat exchanger size and design
- Fan and airflow performance
- Refrigerant circuit design
- Inverter control range
- Ambient and indoor operating conditions
For this reason, comparing systems by their published rated cooling and heating capacity in kW provides a much more meaningful comparison.
Selecting the Correct Air Conditioner
An air conditioner should not be selected by horsepower alone. The required capacity depends on the actual heat load of the space, including factors such as:
- Room size and ceiling height
- Insulation levels
- Window area and orientation
- Local climate
- Number of occupants
- Heat-producing appliances and equipment
- Openings to adjoining rooms or spaces
Summary
Horsepower may still appear in some markets as a traditional product-size description, but it is not an accurate measure of the cooling or heating performance of the complete air conditioning system.
For air conditioners, kilowatts of cooling and heating capacity provide the clearer and more technically meaningful measurement. This is why GENERAL specifies air conditioning capacity in kW.