With the occasionally sweltering heat of summer upon us, you might be eyeing the dusty, creaky air conditioner in the basement and thinking it might be time for a more up-to-date model. Air conditioning, however, is notoriously energy intensive. So, how to choose a unit that will maximize your cool for a minimum cost?

It’s all about the numbers. Here are the figures you need to know:

BTUs: The size of an air conditioner is measured in BTUs, or British thermal units. A small bedroom — say, 12 feet by 10 feet — would need a 5,000-BTU unit, according to the federal energy efficiency program Energy Star; you’d need closer to 14,000 BTUs to cool a more sprawling 20-by-30 room. Very sunny rooms and those regularly occupied by more than two people will require more cooling power than the square footage alone would suggest.

Don’t think you’ll save money on energy use by buying too big or too small for your space. A too-small unit will churn through power without achieving the desired temperature, while a too-big appliance will cycle on and off, resulting in less-efficient energy use.

EERs: The Energy Efficiency Ratio is a simple measure of how much cooling a given air conditioning unit achieves using a certain amount of energy. Higher numbers signal more efficient models; units with budget-friendly EERs of more than 11 are widely available. Find the number on the yellow Energy Guide label displayed in stores and available on online listings.

Estimated Yearly Energy Cost: Exactly what it sounds like, this figure estimates how much a buyer can expect to spend to power the air conditioner over the course of a year and compares it with the operating costs for similar models. The number is calculated using average electricity prices and air conditioning usage, so the actual cost is likely to be somewhat higher in Massachusetts, where power costs are among the highest in the country. Like the EER, the cost estimate can be found on the yellow Energy Guide label.

But must you really do all this math to achieve cool comfort this summer? Consider this: Though only needed for a fraction of the year, home cooling makes up about 6 percent of the average annual energy usage in US households, according to energy.gov. So taking some time to crunch the numbers is definitely worth your while.

Have a consumer question or complaint? Reach Sarah Shemkus at seshemkus@gmail.com.