The wattage needed for an air conditioner to remove a given number of British thermal units varies depending on the efficiency of the unit. The Energy Guide label indicates the efficiency rating. Wattage is determined by dividing the BTUs by the efficiency rating.
In the United States, air conditioners have an Energy Guide label that indicates both the BTUs per hour rating and energy efficiency rating. The energy efficiency depends on the average outside temperature and the inside temperature. Because the standard energy efficiency is calculated based upon an assumed outdoor temperature of 95 F and indoor temperature of 80 F, the efficiency rating is an estimate.
The U.S. Department of Energy standards require that room air conditioners manufactured since 1990 have an energy efficiency of at least 8, while central air conditioners must have an efficiency of at least 9.7. It is common for room conditioners to exceed an efficiency of 10 and central units to exceed 17.