In short
EER2 is cooling efficiency measured at a single hot-day condition: BTU per hour of cooling divided by watts of power, with 35°C outdoors. SEER2 averages a whole season, while EER2 shows what the unit draws at full load in a heat wave. Higher is better, and a unit with a high SEER2 does not always have a high EER2.
EER2 is the efficiency figure that shows how hard an air conditioner has to work on the hottest afternoon of the summer. It appears beside SEER2 on rating sheets and tends to be skipped over. The other ratings are defined in our glossary.
What EER2 measures and how it is calculated
EER2 is cooling capacity in BTU per hour divided by electrical input in watts, measured with the outdoor air at 35°C. A 3-ton unit delivering 36,000 BTU per hour while drawing 3,000 watts has an EER2 of 12.0. At an EER2 of 10 the same cooling takes 3,600 watts. The '2' means the figure was measured under the M1 test procedure, which assumes more duct resistance, so EER2 comes out a few percent below the older EER for identical equipment.
EER2 vs SEER2: a single hot day against a whole season
SEER2 is a seasonal average weighted toward mild weather, and EER2 is one snapshot at full load in the heat. That is why the two can disagree. An inverter unit earns most of its SEER2 by running slowly at part load, and at full output in 35°C air that advantage shrinks. A high-SEER2 inverter model can post a modest EER2 without anything being wrong with it.
- SEER2: seasonal average, the better predictor of the whole summer's cooling bill, explained in what SEER2 means in Ontario
- EER2: one test point at 35°C and full load, the better predictor of peak electrical draw
- CEER: a separate rating for window and room air conditioners, not comparable with either number on a central system
The federal minimum for a split air conditioner, 13.4 SEER2, is a seasonal figure. The history of the seasonal rating is in SEER ratings explained for Ontario homeowners.
Why the hot-day number matters in Ontario
EER2 predicts what your air conditioner draws during a humidex heat wave, which is when it runs longest. The hottest weekday afternoon hours are also when time-of-use electricity prices are at their highest, whichever local utility sends the bill. A unit with a stronger EER2 costs less to run in those hours. ENERGY STAR certification for central air conditioners sets an EER2 minimum alongside the SEER2 minimum, so a certified model has passed both tests.
How to use EER2 on an air conditioner quote
Use EER2 as the tie-breaker. If two units are within about a point of SEER2 and your house runs the air conditioner hard on hot afternoons, pick the one with the higher EER2. West-facing glass, a home office and a hot second floor are the usual reasons. If the system only runs a few weeks a year, purchase price counts for more than either rating.
- Ask for the certified rating of the exact outdoor unit and indoor coil combination, because both ratings change when the coil changes
- Check that the quote shows EER2, not an older EER figure from a discontinued brochure
- Confirm the size first: the wrong AC tonnage costs more comfort and money than a point of efficiency
Central air conditioning runs $3,500–$7,500 installed where the ductwork is already in place, with higher-efficiency models toward the top. ZK Mechanical puts both ratings and the matched model numbers on every air conditioner installation quote. If a quote from someone else leaves EER2 off, ask us to look it up.
Questions people ask about EER2
What is a good EER2 rating for a central air conditioner?
Split central air conditioners sold here commonly fall between roughly 10 and 13. ENERGY STAR certified models sit toward the upper part of that range because the certification includes an EER2 floor. Treat anything in the range as acceptable, and use the number to choose between two otherwise similar units instead of chasing the highest figure available.
Is EER2 the same as EER?
The formula is the same, cooling output divided by power input at 35°C, but the test behind EER2 assumes more resistance in the ductwork. The result is a few percent lower for the same machine. An EER printed on a fifteen-year-old unit's label cannot be compared directly with the EER2 on a new quote, and the old unit no longer performs at its label figure anyway.
Why does a variable-speed air conditioner have a lower EER2 than a cheaper model?
Because EER2 tests the unit at full output, where a variable-speed compressor has no advantage and its electronics add a small loss. The same unit spends most of the summer at reduced speed, which is where it saves electricity and where SEER2 gives it credit. A modest EER2 on an inverter model is normal and is not a sign of a weak product.