Heat wave: We all scream for A.C.! But it pays to keep a cool head before buying a new air conditioner, or cranking the thermostat as low as it'll go. (This won't cool your space any faster, says the Alliance to Save Energy.) Air conditioning represents, on average, about 22 percent of every household's electric bill, according to the Rocky Mountain Institute. You'll save three to five percent on electric costs, plus up to 121 pounds of carbon emissions a year, for every degree you raise the thermostat.

Conventional wisdom advises against buying air conditioners once summer's in full swing and many stores have run out of many current models. But on the bright side, smaller inventories make choices simpler. Here are some simple new and old A.C. tips.

Make sure a new A.C. is Energy Star approved by the U.S. Environmental Protection Agency. To qualify, appliances have to be at least 10% more efficient than conventional ones. For a list of models, go to the Energy Star website and click on Room or Central ACs in the left-hand toolbar. Note, Consumer Reports advises that cheap home room A.C. units can be more practical than central air if only a couple of rooms are in constant use.

It's also worth paying a nominal fee to access Consumer Reports ratings, which balance energy efficiency, performance, size of room the unit can cool, and price.  One current CR best buy is the GE ASM08LK; however, as model numbers can change rapidly, it's best to use CR's picks as a general guideline for what product specs to look for.  Some new efficient models are also mentioned in this article.   

Make sure that you buy a unit with BTUs (British thermal units) appropriate to the size of the space you need to cool. For how to calculate the right BTUs for your square footage, click here.  

Look for room units with an Energy Efficient Ratio (EER) of 10 or more, or central air systems with a Seasonal Energy Efficiency Ration (SEER) of 13 or higher.

We thought they'd gotten rid of these already, but it turns out that many appliances, including a.c.s, still have cooling chemicals that deplete Earth's protective atmospheric ozone layer. Starting in 2010, all new a.c.s will have to have new refrigerants. Some a.c.s that do so are already on the market; Consumer Reports recommends the Haier ESAD4066, $240. For more info, click here.

To make the most of an old A.C., set it as high as you can, ideally at 78 degrees. Keep it at least four degrees higher when you sleep.

When you leave home for a while, turn A.C. off, or raise it by seven degrees.

Clean A.C. filters twice a month, more frequently if it's running every day.

Also see ASE's cool summer ideas, and these "hot tips" from the EPA.

Daydream of permafrost, icebergs and snow.

This article originally appeared in Plenty in July 2008. The story was moved to MNN.com.

Copyright Environ Press 2008