What is EER?
Cooling produced divided by electricity used, at one operating point.
EER is the cooling capacity divided by the electrical power input, measured at full load. A European EER of 3 means three watts of cooling for every watt consumed.
The two scales
They are the same concept in different units, and it is the single biggest source of confusion in the sector. A unit with EER 10 on the US scale has EER 2.93 on the European one. It is not three times worse: it is the same machine measured differently.
Equivalence table
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| European EER (W/W) | US EER (BTU/W·h) | Reading |
|---|---|---|
| 2.2 | 7.5 | Low |
| 2.6 | 8.9 | Poor |
| 3.0 | 10.2 | Average |
| 3.4 | 11.6 | Good |
| 4.0 | 13.6 | Very good |
Using it to estimate consumption
EER lets you go from capacity to consumption, which is what the bill cares about:
A 12,000 BTU/h unit with EER 10 draws 1,200 W. The same unit on the European scale: 3,517 W of capacity divided by an EER of 2.93 gives the same 1,200 W.
Its limitations
- A single point. Measured at full load and standard temperatures. At 104 °F outside, real performance is lower.
- It ignores cycling. It does not reflect the cost of starting and stopping, which on an oversized unit is significant.
- It represents inverters poorly. Those units are most efficient at part load, and EER does not capture that. SEER does.
When to look at EER and when at SEER
Look at EER if you care about behaviour during the hardest hours of summer, with the unit flat out. Look at SEER if you want to estimate a whole season's cost. For most domestic buyers, SEER is the more useful figure.
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 the EER of an air conditioner?
The ratio of cooling capacity produced to electrical power consumed, measured at full load. Higher means less electricity for the same cooling.
What is a good EER rating?
On the US scale, above 11 is good and below 9 is poor. On the European scale, above 3.2 and below 2.6 respectively.
Why are there two EER values?
Because the US expresses it in BTU/h per watt and Europe as a dimensionless W/W ratio. Dividing by 3.412 converts between them.
How do I calculate consumption from EER?
Divide the capacity by the EER using consistent units: BTU/h by the US EER, or watts by the European EER.
EER or SEER when buying?
SEER estimates a whole season's cost better, especially for inverter units. EER is useful for comparing performance under demanding conditions.