BTU stands for British Thermal Unit and is a standardized method of measuring, among other things, the cooling capacity of air conditioners and the heating capacity of furnaces.  

Air conditioners for travel trailer and motorhome are available in three different BTU levels:

• 9,000 BTU - Generally used only in small RVs and where power consumption is an issue.
• 13,500 BTU - Most RV air conditioners are of this BTU rating.
• 15,000 BTU - These larger units are used in long motorhomes or travel trailers of 30 feet or more, usually when there is only one air conditioner to cool the entire unit.