What Is the Difference Between Energy Efficiency and Efficacy?

Ceiling fans don't have energy efficiency ratings; they have efficacy ratings. Since light bulbs are also described by their efficacy, I started wondering about the term.

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 the same type of quantity in the numerator and denominator. That quantity is energy.
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. Same quantity on both sides. That's efficiency.
With ceiling fans and light bulbs, you put energy in and get something else out. With fans, it's air flow, 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 in (watts) and get brightness out (lumens). Look for the ratings on light bulb packages, and you'll see lumens per watt.

In both of these cases where we use efficacy, the output quantity chosen 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.

Repost from Energy Vanguard