When I was doing research for an article on ceiling fans a while back, I noticed that fans don’t have energy efficiency ratings; they have efficacy ratings.

There’s certainly confusion about the terminology among different sources, but since light bulbs are also described by their efficacy, I started wondering about the term. I’d just accepted it before, with a vague understanding that there was something different about how efficacy was defined. Now I know why.

How much bang for your buck?

Here’s the deal. Efficiency in general is defined as output divided by input. It’s a number that tells you how much bang you get for your buck.

By itself, though, that definition doesn’t distinguish between efficiency and efficacy because the latter is also a bang-to-buck ratio. Here’s the difference: An efficiency rating has an energy value in both the numerator and denominator, whereas an efficacy rating has an energy value in the denominator but something else in the numerator.

Take a look at furnace efficiency, for example. The official efficiency rating is called Annual Fuel Utilization Efficiency, or AFUE. It’s the number of BTUs of heat that can be delivered to the heated space divided by the number of BTUs of heat that result from burning the fuel. A typical furnace might have an input rating of 80,000 BTU per hour and have an AFUE of 80. That means 64,000 BTU of heat per hour get delivered to the house.

Efficacy is different but the same

You put a certain amount of energy in, and you get a certain amount of energy out. We’re talking about energy output as well as energy input. That’s efficiency.

With ceiling fans and light bulbs, however, you put energy in and get something else out. With fans, the output is the air flow rate, measured in cubic feet per minute. The rating that you see on ceiling fan boxes now shows cfm per watt.

With light bulbs, you put energy (watts) in and get brightness (lumens) out. Look for the ratings on light bulb packages, and you’ll see lumens per watt.

In both of these cases involving efficacy, the type of output chosen (air flow rate or lumens) makes more sense to look at than does energy. With heating and cooling, it makes more sense to talk about efficiency because we want to know about the energy output, and we can measure it relatively easily.

Now you know the difference. You can use the word “efficacy” and look smart. And you’ll look even smarter when someone asks why you’re using that word instead of efficiency.

Allison Bailes of Decatur, Georgia, is a speaker, writer, energy consultant, RESNET-certified trainer, and the author of the Energy Vanguard Blog. Check out his in-depth course, Mastering Building Science at Heatspring Learning Institute, and follow him on Twitter at @EnergyVanguard.