©2013 Green Building Advisor. From The Taunton Press, Inc., publisher of Fine Homebuilding Magazine.
A little over a decade ago when I was building a house  and buying a bunch of ceiling fans, it wasn't so easy to figure out which fans were energy efficient and which weren't. That's not the case anymore because every ceiling fan now has a label on the package that tells you how much air movement you can expect for each watt of electricity you put into the fan.
Air flow, whether you're talking about duct systems, bath fans, heat recovery ventilators, or ceiling fans, is measured in cubic feet per minute (in the U.S., of course, where we use those annoying imperial units ). We usually abbreviate and simply call it cfm. The higher the number, the more air is moving.
Energy usage can be a bit confusing because of the whole rate versus cumulative quantity  issue. The watt is the unit for the rate of energy consumption. The kilowatt-hour is the unit for the quantity of energy consumed.
Efficiency of any type is generally an output divided by an input. If we are comparing an energy input with an energy output, we report the result as an efficiency rating. If the output we are looking at is not an energy output, however, we use the term “efficacy.” Heating and cooling system ratings generally refer to efficiency: AFUE, SEER, HSPF...  On the other hand, light bulb ratings compare brightness (the output) with energy (the input); thus we speak of light bulb efficacy, not efficiency.
Keeping Cool 
Hot-Climate Design 
To measure how well a ceiling fan does its job, the output that we are measuring is not an energy output; it's air flow. So, as with light bulbs, we’re talking about efficacy, not efficiency.
We can go with either rates or quantities when we look at efficacy of ceiling fans. For example, we could give the efficacy in terms of the rate of air flow per unit time (cfm) divided by the energy use per unit time (watts), which in fact is what we do. Most ceiling fan packages show a box with three numbers, all specified for the fan running at high speed: the air flow (cfm), the rate of electricity use (watts), and the air flow efficacy (cfm/watt). You can see this in the labels shown below.
I went down to the big box store a while back and started looking at ceiling fan packages. The range of efficacies that I saw in the store ran from 35 cfm/watt on the low end to 106 cfm/watt at the high end. Most of the fans were in the 60 to 75 cfm/watt range. One factor that affects this number is how big the fan is. That 35 cfm/watt fan was a tiny 24", whereas the 106 cfm/watt fan was 52". The range of efficacies for the larger fans, according to the label shown below, is 51 to 176 cfm per watt for fans that are 49" to 60".
A few of the boxes had more energy data than the simple box shown above. They also happened to be the fans that had the Energy StarLabeling system sponsored by the Environmental Protection Agency and the US Department of Energy for labeling the most energy-efficient products on the market; applies to a wide range of products, from computers and office equipment to refrigerators and air conditioners. label. In addition to showing the numbers for high speed, they also showed the air flow, energy consumption, and efficacy for low and medium speeds.
If you look carefully at the two labels below, you'll see one other thing there that I saw on nearly every label: a money saving tip. Here it is: “Turn off fan when leaving room.” That's right. Fans don't cool the space. They cool people. They actually heat the space because the motor gives off heat, and the stirring up of the air results in heating, too. So, turn off those ceiling fans! 
Choosing a fan with a high efficacy is only part of what you need to do to get an energy efficient ceiling fan. I bought a new ceiling fan recently and noticed that many of them gave sizing recommendations. And many of those recommendations were not ideal.
All else being equal, efficacy increases when you increase the blade length. Think about it. A motor causes the blades to move through the air. Because of the way they're slanted with respect to the plane of rotation, they push air. To get more air movement, you can do two things (assuming you’re not changing the room or fan position): Increase the fan speed or increase the blade size. Both increase the energy usage, but increasing the fan speed increases the energy usage a lot more than increasing the blade size.
What that means, if you're looking for a fan that does its job as efficiently as possible, is that you should get a bigger fan than some of the boxes recommend. To make matters even more confusing, the Energy Star page on ceiling fan sizing  recommends installing smaller, less efficent fans than you really should. You can see their sizing chart in the images below.
If I'd gone with their recommendation, I would have bought a 44" ceiling fan since the bedroom I installed it in is about 160 square feet. The efficacy for a typical 44" ceiling fan at that store was about 60 cfm/watt at high speed. The 52" fan I bought gets 81 cfm/watt at high speed.
Even better is that I can run the 52" fan at medium speed to get about the same air flow as the 44" fan at high speed. The efficacy comparison then is 117 cfm/watt for the 52" fan versus 60 cfm/watt for the 44" fan. That means I'll use about half the energy to get the same air flow. In addition to saving energy, fans are also quieter when they run at lower speeds.
If you want to stick to Energy Star certified ceiling fans, you need to go to the Energy Star website and download their ceiling fan spreadsheet  (look for the link in the sidebar on the Energy Star website). The Excel format allows you to sort by brand, size, efficacy (at high, medium, or low speed), or any of several other parameters. For example, if you know you want a 52" ceiling fan with a light kit, you can sort the whole list to view only those that meet your criteria. It’s a great resource.
One interesting thing you’ll find when you sort is that the most efficient Energy Star ceiling fans are made by a company with the audacious name of Big Ass Fans . Their Haiku residential fan, shown below, is gorgeous and has efficacies that range from 846 to 1018 cfm/watt on low speed and 422 to 549 cfm/watt on high speed. They blow most of the others away! (Ha ha ha.)
Another brand that comes close is Aeratron , with its E502-SL and E503-SL models. They come in at 608 to 655 cfm/watt on low speed and 357 to 383 cfm/watt on high speed. If you’re looking for the really high efficacy ceiling fans, though, you probably won’t find them at your local big box store. You also won’t find them at big box store prices.
If you want to get the most out of your ceiling fans, there are only three simple rules you need to know:
On the sizing recommendation, just make sure you meet all the specified clearances. Then you can get the biggest one that makes sense for the room.
Do your research, follow these three rules, and your new ceiling fans will help you be more energy efficient in your home, not less.
Allison Bailes  of Decatur, Georgia, is a speaker, writer, energy consultant, RESNET-certified trainer, and the author of the Energy Vanguard Blog . You can follow him on Twitter at @EnergyVanguard .