When shopping for AC, you’re going to see a few strange numbers: EER and SEER. These are you Energy Efficiency Ratio and your Seasonal Energy Efficiency Ratio. We use these to understand how well a given air conditioner performs, how much power does it take to keep you cool.

### Getting the Ratio

The math behind an EER ratio is pretty straight forward: BTU/Watts. This is essentially miles per gallon, but for an air conditioner. You could think of it as coolness per watt. For example, we could have a theoretical 6000BTU window unit that draws 300 watts. 6000/300 gets us 20. It’s just another number, right?

The useful factor comes in with comparison shopping. We know that particular unit is going to be extremely efficient. The manufacturers already do the calculations for us, so it’s just a matter of finding the EER rating on the box or online documentation. We can look around other units, and maybe we’ll see a few 21’s, maybe some 18’s and probably quite a few cheaper units with a 13 EER.

The rule of thumb is to go for the highest EER you can afford, as it will save you more money later. That does however, beg the question of how much money. That does require some extra math.

Machine 1 has an EER of 20. It runs on 300 watts. 300 x 24 hours = 7200 watts for 1 day of continuous use.

Machine 2 has an EER of 12. It runs on 500 watts. 500 x 24 hours = 12,000 watts for 1 day of continuous use.

Depending on locality, that could be as much as $2.50 more per full-day of running. These numbers add up fast.

### Numbers Aren’t Everything

There is a caveat in this, a grain of salt we need to keep on our shoulders: these are numbers we’ll almost never see in real world running. Your car might hit 30 MPG on the highway, but to do that, you need the right weather conditions and probably a cruising speed of 45mpg. That is to say, an EER on its own is only a rough idea of what to expect.

It’s also important to remember: you’re not going to pull down 300 watts/hour unless the air conditioner is always cycled to the on state. It needs 300 watts, maybe a bit more when the compressor runs, but the blower motor that runs the other 90% of the time, does not need so much power.

### A Seasonal Average

These numbers so far are specific peaks about what a system can do, but they don’t account for all the varied weather and conditions of a long summer. They’re just a best-case scenario. Central cooling systems are generally rated with a SEER, Seasonal Energy Efficiency Ratio. This is a more nuanced number, derived from the ups and downs of a whole cooling season. It’s an average of performance. You might draw 300 watts/hour one day, and only 200 watts/hour the next.

With this average, you get a better idea of what the real-world delivers. For something as expensive and complicated to install as a central cooling system, you want the most accurate picture, not the ideal picture. The SEER is still an optimistic number, but as an average, it is weighed down and more balanced.

### Follow the Numbers

Air conditioning is very nearly a need in our roasting hot, modern world. It’s also a feat of engineering with well documented concepts of performance and cost. Always follow the numbers when picking the system that’s best for you. It’ll save money in the long run.