The rated capacity of Air Conditioning (A/C) units and Heat Pumps is primarily expressed in BTU (British Thermal Units) rather than kilowatts (kW) for several reasons. The reasons and international standards that influence this choice are detailed below.

1. History and Tradition:
• The BTU measurement unit has historically been used in the air conditioning and refrigeration industry. Therefore, it is a widely recognized and accepted unit of measurement throughout the world.
• HVAC and refrigeration equipment manufacturers have been using the BTU unit as a reference since 1956. Therefore, this has created a deep-rooted tradition in this industry.

2. Global Market:
• The global air conditioning and heat pump equipment market is diverse and spans different regions with different standards and preferences.
• By expressing rated capacity in BTU, manufacturers can serve a broader audience and facilitate comparison between different models and brands.

3. International Standards:
There are international standards that influence the expression of nominal capacity. For example:
• ISO 5151: This standard establishes the methods for determining the cooling and heating capacity of air conditioning and heat pump systems. Although it allows the capacity to be expressed in kW, it also mentions the option of using BTU.
• ASHRAE 90.1: This American Society of Heating, Refrigerating and Air-Conditioning Engineers (ASHRAE) standard is widely used throughout the world. Although it is based on International Units, like kW, it also provides equivalent BTU values to facilitate overall application.

4. Conversion and Comparison:
• The conversion between BTU and kW is simple but not always intuitive for consumers. Therefore, most of manufacturers prefer to keep the measurement in BTU to avoid confusion.
• The BTU rating allows users to easily compare different models and evaluate their energy efficiency.

5. US Market Size:
The US market is one of the largest for air conditioners and heat pumps. Since American consumers are more familiar with BTU, most of manufacturers have choose to keep this unit of measurement and give the kW a secondary place.

In summary, the choice between BTU and kW, to express the nominal capacity of Air Conditioning and Heat Pump equipment, is a combination of Tradition, International Standards, Global Market and Ease of Comparison by the Consumers.

Although the trend of Standard Organizations is towards using International Units, such as the kW, nowadays the industry continues to use the BTU to meet the needs of a diverse and global consumer’s audience.

If you are looking for a Hydronic based HVAC Systems, you can visit our website: where you'll find all the necessary information -in BTU & kW- to choice the Air-to-Water Heat Pump more suitable for your home.