Earlier quoted context omitted.
There are standardized efficiency measurements that all A/C manufacturers must conduct and publish in the US. You'll see terms like COP, EER, and SEER. Those allow you to compare pretty easily. Currently the minimum is a SEER of 13, but I've seen units as high as 22 advertised.
Higher is better in this case?
EDIT: I should mention that SEER uses different units for each part of the ratio -- BTUs of heat moved vs watt-hours expended. COP is dimensionless, and EER is just SEER but for a specific operating point. SEER extends the measurement through an entire "season", and so it theoretically gives a more realistic picture of overall efficiency.