BTU stands for British thermal unit. It is a traditional unit of heat that defines the amount of heat that is needed to raise the temperature of one pound of water by one degree Fahrenheit, One BTU is equivalent to roughly 1055 joules. In relation to air conditioning units, the measurement is used to express the amount of heat that can be added or removed from the surrounding air. Generally, the higher the BTU the quicker the air can be cooled or heated, however, when choosing an RV air conditioner, it is important to weigh this against the size of the area you need to cool or heat. If the unit is too powerful it could cool the area too quickly and leave you either freezing or constantly adjusting your thermostat and temperature controls.