The easiest place to look is on the air conditioner itself. Go outside to the condensing unit and look for a data plate mounted to the side. Find the model number. Within this string of letters and numerals, you should find an even, two-digit number. The possibilities on residential units range from 18 to 60. Divide the number by 12 (which represents 12,000 Btu/hr, or one ton of cooling capacity) to get your AC unit’s tonnage.

If you don’t want to do the math, here are the numbers you’re looking for and their corresponding tonnage:

18 = 1.5 tons

24 = 2 tons

30 = 2.5 tons

36 = 3 tons

42 = 3.5 tons

48 = 4 tons

60 = 5 tons

Heating and air conditioning capacities are measured in British thermal units (Btu) per hour. One Btu is equivalent to the heat generated by a single birthday candle. It’s the amount of heat needed to raise the temperature of one pound of water by one degree.

The word “tonnage” is used to describe the cooling capacity of an air conditioner. One ton of cooling is the rate of heat transfer needed to freeze 2,000 lbs – or one ton – of water in 24 hours. This is equivalent to 12,000 Btu/hr.

In some cases, bigger is better. This is certainly true of meal sizes, lottery winnings, and discounts. But when it comes to air conditioners, sometimes less is more. You should never simply buy the biggest AC unit possible or you could end up with an oversized system. Here are the problems associated with oversized air conditioners:

• Short cycling leads to increased wear and tear on the unit, resulting in more repairs and premature replacement.
• Short cycling prevents dehumidification, which leaves you with a cool but clammy interior. You may find yourself turning the temperature down to combat this, raising your electricity usage in the process.
• Bigger air conditioners cost more to purchase and operate, a needless expense if your home is eligible for a smaller system.

Of course, an undersized unit is also problematic because it can’t keep up on the hottest summer days.

SEER  stands for Seasonal Energy Efficiency Ratio. This is the ratio of the cooling output of an air conditioner over a typical cooling season, divided by the energy it uses in Watt-Hours. It may also be called a Seasonal Energy Efficiency Rating.

A SEER ratio is calculated over an entire cooling season using a constant indoor temperature and a variety of outdoor temperatures ranging from 60 degrees to 100 plus. This is how it simulates a typical season.

Keep in mind that SEER ratio is a maximum efficiency rating, like the miles per gallon for your car. Say your car gets 28 miles per gallon on the highway. But if you’re stuck in city traffic it’s a lot less efficient. The same goes for your air conditioner. If your SEER ratio is 21, that’s the maximum efficiency and it could be lower depending on conditions.

A higher SEER rating provides greater energy efficiency in certain conditions. The minimum standard SEER for air conditioners is 13, though most modern air conditioners have a SEER that ranges from 14 to 21. The efficiency of your system can vary based on the size of your home, your current ductwork and other variables

The U.S. Department of Energy enforces minimum SEER requirements that differ by geographical region. The minimum in the Southwest and Southeast is 14 and it’s goes down to 13 in the North. A 13 or 14 SEER rating doesn’t necessarily mean a unit is inefficient. Most older A/C systems are rated at around 8 or 9, so even the lowest available SEER rated system you buy today will be much more energy efficient.

Getting an air conditioning system with a higher SEER does mean you’ll be more comfortable in the summer months.

Higher SEER units often have 2 components that provide greater indoor comfort.

• 2-stage or variable-speed compressor
• Variable-speed blower

Air conditioners with lower SEER ratings are usually single-stage and only run on one speed. This means they’ll frequently turn on and off during mild weather and you’ll experience uneven cooling or hot and cold spots. You’ll also experience higher humidity levels which makes it feel hotter than it is. Your A/C needs to run for a long period of time to remove humidity from your home’s air. The ups and downs of a single-stage system don’t accommodate for this.

There’s no magic SEER number. Anything over 14 is great. Because if you have an old 8 SEER system and replace it with a 16 SEER unit, you could significantly reduce the cost of cooling your home.