They use a lot of energy when they turn on initially, after that, its very little . Ive been working on smart thermostats that take advantage of this phenomenon to lower HVAC costs.
AFAIK, the most efficient residential systems are ductless split systems, where you basically have a centralized compressor and distributed air handlers in individual rooms. This allows for variable flow and localized control, which reduces need.
[0] https://en.wikipedia.org/wiki/Seasonal_energy_efficiency_rat...
> But when either replacing equipment, or specifying new installations, a variety of SEERs are available. For most applications, the minimum or near-minimum SEER units are most cost effective, but the longer the cooling seasons, the higher the electricity costs, and the longer the purchasers will own the systems, the more that incrementally higher SEER units are justified.