What is a SEER rating?

The one-line definition

SEER stands for Seasonal Energy Efficiency Ratio. It is the cooling an air conditioner produces over a whole season divided by the electricity it consumes doing it. Higher is better.

This Old House's guide hands the explanation to an expert from its own television series, and we have not found a better one:

He compares SEER2 ratings to miles per gallon in your car, which is a simple way to understand that a higher number means better performance for every energy dollar you spend.

That analogy carries further than it first appears, and the rest of this page is mostly about one way in which it does.

Why your quote may show two different numbers

In 2023 the industry changed how the number is measured. This Old House puts it plainly: as of 2023, SEER2 became the new standard efficiency rating system for air conditioners and heat pumps, and SEER2 uses stricter testing conditions that better reflect real-world operating environments.

Stricter test conditions produce a smaller number. The machine did not get worse. The test got harder. Fixr says so directly and gives the size of the effect:

It is known as SEER2, and the new numbers will be 4.5% lower than the old numbers. So, while your new unit may be more efficient than the old, it may look like it has a lower efficiency rating.

Fixr adds that most manufacturers list both numbers on the label for exactly this reason.

The miles-per-gallon analogy again, and this is our point rather than a source's. This is the same thing that happened to car fuel economy when the test cycle was revised. The car that used to be rated at 30 mpg is now rated at 28, and it is the same car. Nobody should pay more or less for it on that basis. The only mistake is comparing a number from the old test against a number from the new one.

The conversion table

This is the part worth printing out. Fixr's central air guide lists both scales against each other, with what the unit alone costs at each step:

We checked the 4.5% claim against the table rather than taking it on trust, and this arithmetic is ours. Every pair in that table falls between 4.29% and 4.76% lower, and the pair at 20 SEER lands on 4.5% exactly. So the rule of thumb holds across the whole range, and a rough conversion in your head is good enough: knock about one point off a mid-range SEER number and you have the SEER2 number.

That also means something slightly awkward. A 14 SEER machine and a 13.4 SEER2 machine are not two choices. They are one machine described twice. If a contractor quotes you both, you are not looking at an upgrade.

The minimum you are allowed to buy, by region

Here the two publishers describe the same rule in the two different scales, which is the clearest demonstration we can give that the scales are interchangeable.

This Old House, in SEER2: homes in northern states usually require cooling units with a 13.4 SEER2 rating or higher, while homes in Southeastern and Southwestern states often require 14.3 SEER2 or higher.

Fixr, in SEER, on the same two tiers: 14 SEER is "the new minimum standard for homes located in northern climates", and units of that rating "cannot be installed in moderate or hot climates after January 1, 2023". 15 SEER is "the new minimum standard SEER rating for AC units used in moderate-to-warm climates".

Line those up against the conversion table and they are the same two numbers:

Our note, marked as ours. Two independent guides, two different scales, no disagreement at all once converted. That is rarer than it should be in this subject, and on this site it is worth saying when it happens. Our central air page covers what the systems themselves cost, and our what size air conditioner page covers the other half of the specification on your quote.

How high the numbers go

This Old House: today's high-efficiency units can reach a rating of 26 or higher SEER2.

Fixr's central air table stops at 21 SEER, which is 20 SEER2, and it calls 21 SEER "one of the highest recommended ratings for residential use", qualified with "mostly in extremely hot climates and only if you plan on living in the home long-term to make up the cost difference".

Our reading, marked as ours. Those two statements are not in conflict. One is about what exists, the other about what pays for itself. A 26 SEER2 machine is buyable. Whether the extra efficiency earns back its price depends on how many hours a year you run it, which is the whole reason the recommendations are written by climate rather than as a single number.

What SEER does not tell you

SEER is a cooling rating only. Fixr's heat pump guide names the division: SEER and SEER2 measure cooling, while HSPF (heating seasonal performance factor) measures heating. A machine that heats as well as cools therefore carries two ratings, and our how a heat pump works page explains why neither can be compared against a furnace's AFUE percentage.

Fixr prices heat pumps on the pair of them:

What a high rating is worth in money

Fixr's heat pump guide is the only one of our three sources that works the running cost all the way through, and it is careful to label it an example rather than a prediction. For an 1,800 square foot home with a 5-ton heat pump at SEER2 18, running 4 hours a day, at the national average electricity price it gives as 17.47 cents per kilowatt-hour, the cost is about $1,200 annually, which it describes as roughly $100 a month for both heating and cooling.

Our note, marked as ours. Every input in that sentence is a variable, and the electricity price is the one that varies most between states. Treat it as a method you can repeat with your own kWh price rather than as a figure that applies to your house.

On the purchase side, Fixr's own headline specification is instructive about how much the rating moves the total. The $8,750 average central AC installation is a 3-ton split system at 16 SEER with new ductwork in a 2,000 square foot home. The $3,750 low end is a change-out of a 3-ton unit at 14 SEER using existing ducts. The $18,000 high end is a 3-ton unit at 21 SEER with 300 feet of new ducts and customisations.

Our reading, marked as ours. All three are 3-ton machines. The rating moves between 14 and 21, but so does the ductwork, and the ductwork is doing most of the work in that spread. The efficiency rating is a smaller lever on the installed price than the table of unit prices makes it look, because the unit is only part of the bill. Our central air page takes that apart line by line.

One table we would not rely on

This Old House prints a cost table by rating and home size, credited to Homewyse. We are flagging it rather than reproducing it, for three reasons, and these observations are ours.

The first is cosmetic: the table's column headings read SEER 15, SEER 16 and SEER 17 while the sentence introducing it says it outlines costs based on SEER2 ratings. Given everything above, those are not the same columns. We cannot tell from the page which scale the figures use.

The second is arithmetic. At 1,000 square feet the figures are $7,376 at SEER 15 and $7,333 at SEER 16. The more efficient machine is listed as the cheaper one, by $43.

The third is the pattern down the rows: the 1,100 square foot row and the 1,200 square foot row are identical at $7,479, $7,436 and $9,535. A table that does not move between those two sizes is not really answering by size.

None of that makes This Old House's prose wrong, and we have used its definitions and its regional minimums throughout this page. It does mean we would not quote that particular table at anybody, and we would rather say so than reprint it.

What to take from this

1. SEER is miles per gallon for an air conditioner. Higher number, less electricity for the same cooling.
2. SEER2 is the same thing measured under a harder test. Expect the number to be about 4.5% lower for the identical machine.
3. Check which scale a quote is using before comparing two quotes. 14 SEER and 13.4 SEER2 are one machine, not two options.
4. Your region sets a floor, 13.4 SEER2 in the north and 14.3 SEER2 in the Southeast and Southwest, and below that the machine cannot legally be installed.
5. Higher is not automatically worth it. The payback is a function of how many hours a year the machine runs, which is why every recommendation is written by climate.
6. SEER says nothing about heating. If the machine heats too, ask for the HSPF as well, which our heat pump page and our heat pump vs furnace page both come back to.