What is SEER?
Efficiency across a whole season, not at a single moment.
SEER is the total cooling produced over a season divided by the total electricity used in that same season. Unlike EER, it includes start-ups, stops and part-load running.
The formula
A unit producing 10 million BTU over a summer while using 500 kWh has a SEER of 10,000,000 / 500,000 = 20. On the dimensionless European scale that is 5.86, dividing by 3.412.
SEER against EER
Swipe to see the whole table
| EER | SEER | |
|---|---|---|
| What it measures | One operating point | A whole season |
| Conditions | Full load, standard temperature | A profile of temperatures and loads |
| Includes cycling | No | Yes |
| Reflects inverters | Poorly | Well |
| Typical value | Lower | Higher for the same unit |
Comparing one unit's SEER with another's EER is meaningless: they are different measurements. Compare SEER with SEER.
Why SEER exists
An air conditioner almost never runs at EER test conditions. It spends most of the summer at part load, cycling on and off, and with an inverter unit, modulating. EER reflects none of that; SEER does.
Which is why SEER predicts your bill far better.
SEER2, since 2023
US labels have used SEER2 since 2023, measured at a higher external static pressure to better represent a real ducted installation. The same unit shows a slightly lower SEER2 than its old SEER: it has not got worse, it is measured differently. Do not compare a SEER2 against an older SEER without allowing for that.
Using it when buying
A higher SEER means less electricity for the same cooling, but also a higher purchase price. The calculation that matters is how many years the saving takes to repay the difference, and that depends on how many hours you will run it. In a holiday home used occasionally, paying a lot more for a high SEER rarely pays back.
Sources and methodology
Conversions use exact constants: 1 BTU/h = 0.29307107 W and 1 frigorie/h = 1 kcal/h = 1.163 W. The component sizing model (load per volume by insulation level, 120 W per person, solar gain by glazed area) is set out in full on the air conditioner size calculator. It is a guideline estimate: a professional heat-load calculation considers considerably more variables.
Frequently asked questions
What is a good SEER rating?
It depends on the market and current regulations. For modern domestic units, high performers sit at or above 20 on the US scale.
What is the difference between SEER and EER?
EER measures one point at full load; SEER averages a whole season including cycling and part load. SEER is usually higher for the same unit.
What is SEER2?
The US test procedure in force since 2023, using a higher external static pressure. It gives slightly lower values for the same equipment.
Is a higher SEER worth the extra cost?
It depends on hours of use. With heavy use the saving repays the difference; with occasional use it rarely does.
How do I calculate SEER myself?
Divide the seasonal cooling output in BTU by the seasonal electricity in watt-hours. The SEER calculator does it both ways.