Both MMBTU and MMBtu are abbreviations for ‘Million British Thermal Units’. One British Thermal Unit (BTU and Btu) is the amount of heat required to increase the temperature of one pound of water by one degree Fahrenheit. The BTU and the MMBTU are recognized as units of energy in the United States system of customary units. The BTU is often used to quantify the cooling capacity of an air conditioning system. A wooden kitchen match produces approximately 1 BTU, and the cooling capacity of household air conditioners is typically between 5,000 and 15,000 BTU per hour.