The hourly operating cost for a 12,000 BTU air conditioner depends on factors such as the unit’s Energy Efficiency Ratio, "EER," or Seasonal Energy Efficiency Ratio, "SEER," room or house size and local energy rates. As of 2014, the average cost for energy is nearly 13 cents per kilowatt-hour, "kWh."

To calculate the hourly cost of a 12,000 BTU unit, divide BTUs by its EER or SEER to get watts per hour. Divide the watts per hour by 1,000 to find kWhs. Multiply the kWhs by the local electric company’s rate to calculate the cost per hour. For example, if the unit has an EER of 9, the average hourly cost of operation would be 17 cents.