SEER2 ratings explained (and when a higher one pays back)

SEER stands for Seasonal Energy Efficiency Ratio. The US Department of Energy defines it as the cooling output of a system divided by its overall power consumption during the normal cooling season. The higher the rating, the less electricity the system uses to deliver the same cooling.

Think of it like miles per gallon. It is measured under set test conditions, and your results depend on how and where you drive. A rating is a fair way to compare two units. It is not a promise about your bill.

SEER2 is the rating you will see on equipment sold today. It measures the same thing as SEER with an updated test procedure. Because the test changed, a SEER2 number and an older SEER number for the same unit are different, and you cannot compare them directly.

The practical rule: compare SEER2 with SEER2. If one quote lists SEER and another lists SEER2, ask the contractor to restate both on the same scale. Federal minimum ratings vary by region and equipment type, so ask each bidder to confirm that the quoted unit meets the minimum for your state.

A more efficient unit costs more up front and less to run. Whether the difference comes back to you depends on four things:

• Cooling hours. In a long, hot season the AC runs a lot, so each step up in efficiency saves more.
• Your electricity rate. The higher the rate, the more each saved kilowatt-hour is worth.
• The price gap. Ask for the installed price at two or three efficiency levels from the same contractor.
• How long you will stay. Savings arrive year by year. If you plan to move soon, you may not collect them.

To estimate payback, ask the contractor for the expected yearly cooling cost of each option at your electricity rate. Divide the extra installed price by the yearly savings. The result is the number of years to break even. The guide to AC replacement cost covers how efficiency changes the price.

The rating on the label assumes a correct installation. A high-efficiency unit that is the wrong size, has the wrong refrigerant charge or is connected to leaky ducts will not deliver its rated performance.

Size comes first. The Department of Energy says a contractor should use a Manual J load calculation to size the equipment, and warns that oversized cooling systems can cost too much and provide inadequate comfort. The AC size calculator gives you a rough check on the tonnage in a quote.

Higher-rated equipment often comes with two-stage or variable-speed operation. Those features can improve comfort and humidity control, which some owners value more than the savings.

Replacing a working AC only to gain efficiency is a harder case to make than choosing a better rating when the old unit is already failing. ENERGY STAR suggests you consider replacement once an AC is more than 10 years old, and says a correctly installed ENERGY STAR unit can save up to 20 percent on heating and cooling costs. Whether that covers the cost of a new system depends on your bills. The repair or replace calculator weighs age and repair costs together.