John R
energy star air conditioners costs Google above- lots of sites, esp. from Fed. Gov. & cost calculators.

BW
I researched this exact topic this summer when I wanted to purchase a new Energy Star AC window unit. What I learned is that you want something with an energy efficiency rating close to 11. (Higher is better; however, there's no sense in buying more air conditioner than you need - buy one that is recommended to treat the square footage of your space.) I ended up buying an LG from Home Depot. I love it! Although the online ratings are mixed, my experience is that it's very quiet for the size unit (15,000 btu is on the larger side for a window unit), very good at cooling the large space that I have, and very energy efficient. It's a little bit tricky to install only because it is a full-size unit and the brackets that come with it won't necessarily fit vinyl replacement windows (I used a separate window AC mount), but I am VERY pleased with my purchase.