In terms of energy efficiency rating (EER), window-unit air conditioners are slightly less efficient than central air. Window units that are certified Energy-Star only need to score between 9.4-10.7 EER, while Energy Star-certified need to score 12 EER or above.
Central air conditioners tend to dehumidify at a faster rate, making the air feel colder even if the same temperature can be achieved with a window unit. The advantage of window units over central air is that central air systems suffer from "duct loss," which means that the conditioned air heats up and leaks as it passes through the ducts of a house. Overall efficiency depends on the particularities of the house, how many rooms are cooled and the brand of air conditioner.