If you’re shopping for a new HVAC system, the price will likely figure heavily in your decision, but there’s another number you should pay close attention to: the SEER rating.

That’s the Seasonal Energy Efficiency Ratio. It’s a performance measurement calculated by the system’s cooling output over the course of a summer divided by the energy it uses in total watt-hours during that same time period. Units on the low end of the SEER scale have a minimum rating of 13, whereas the most efficient models can achieve a SEER of 25. (That said, a modern system with a low SEER rating is significantly more efficient than the older model you’re looking to replace.)

What makes a SEER A/C system different?

A modern air conditioner with a high SEER has a variable-speed compressor and blower. That means the system operates on a range of speeds, from low to high, depending on the cooling needs of your home. For example, the HVAC might begin its cycle at a high speed to drop the temperature, then gear down to a lower setting to maintain that temperature for longer intervals.

In contrast, a fixed-speed system kicks on with a high blast of air and shuts off when it reaches a desired temperature. These on-off cycles drive up energy consumption.

Think of it this way: Your car is more efficient on the open road than in stop-and-go traffic. A high-efficiency HVAC system operates on the same principle.

Beyond energy savings

Other than a lower energy bill, you’ll also experience a more consistent temperature throughout the home. High-SEER systems are quieter. And because they run longer, they take more humidity out of the air. (Humidity can make it feel hotter than it is.)

Optimize efficiency

Just because you’re buying the most efficient model on the market, that doesn’t mean you should depend on it to do all the work. Augment your air conditioning with fans. With fans, you can bump the thermostat up 4 degrees without sacrificing comfort.

And if you have a programmable thermostat, set it to a warmer temperature while you’re away and schedule it to begin a cooler cycle before you get back.