Here's some advice -- don't wait too long because you don't want to do your shopping when the mercury hits 90 or above. By then, everyone will be trying to buy an air conditioner and the shelves at even the big box stores may be picked clean.
The American Home Appliance Manufacturers Association (AHAM) has some more advice for picking an air conditioner that will do the job without running up the electric bill more than necessary. For starters, measure the square footage of the area you want to cool. Air conditioners are rated, not just in terms of BYU, but also in the square footage they can optimally cool.
While you're at it, measure the windows in the room you want to cool. Both portable and room air conditioners require connection to a window. Whatever you end up buying, make sure it's going to fit. It's less of an issue with a portable unit but very important for a room air conditioner that actually sits in a window.
Consider other factors
Consider other factors influencing the area you want to cool. You may have a 600 square foot room and the air condition is rated to cool that size space. But If the room has a lot of southern glass exposure, it's going to get a lot of solar heat, making any air conditioner work harder. In that case, it might be cost effective to move up a size, choosing a model rated to cool a larger space.
Generally, however, a space 150 to 350 square feet will need a 5,000 to 8,000 BTU air conditioner. From 350 to 550 square feet, look for an 8,000 to 12,000 BTU unit. Very large spaces, from 1,050 to 1,600 square feet, will need a very large capacity unit, rated from 18,500 to 25,000 BTU.
Smart features
Even some portable and window units now offer smart technology like you find on central air units. Some have something called a "follow me" function that measures the temperature, both at the unit and at the location of the remote control. Some models may also feature programmable timers and alerts to remind you to change the filter.
Some models can operate in different modes. For example, the cooling mode takes in room air, cools it and directs it back into the room. Some models also let you select from several preset cooling levels. Some let you pick a temperature and the unit keeps it there.
A fan-only mode uses less energy because the condenser stays off, meaning it doesn't draw nearly as much electricity.
Learn more in the ConsumerAffairs Air Conditioning Buyers Guide.