EER Rating Explained: EER vs EER2 and Why Peak-Heat Efficiency Matters

What EER measures

Energy Efficiency Ratio is the simplest efficiency number in cooling. Run the air conditioner at full capacity under fixed test conditions, measure the cooling it delivers and the electricity it draws, and divide one by the other.

EER = Cooling output (BTU/h) ÷ Electrical input (W)
Input (W) = Capacity (BTU/h) ÷ EER

For central systems the standard rating point is 95 °F outdoor dry-bulb, with return air at 80 °F dry-bulb and 67 °F wet-bulb (about 50% relative humidity). Because it is a single steady-state point at full load, EER does not reward variable-speed compressors for running slowly in mild weather. That is the job of SEER2.

A 3-ton (36,000 BTU/h) unit rated 11 EER draws about 36,000 ÷ 11 = 3,270 W at the rating point. At 13 EER it would draw about 2,770 W, around 500 W less every hour it runs flat out.

EER vs EER2

The 2023 DOE test procedure (M1) raised the external static pressure used in testing from roughly 0.1 to 0.5 in. w.c. for typical ducted systems, so the indoor blower uses more power and the measured ratio falls. The temperatures stayed the same. As with SEER, the new number reads a few percent lower for identical equipment, so compare EER2 only with EER2. When a spec sheet lists both, use EER2 for any post-2023 comparison or rebate form.

SEER2 vs EER2: typical pairings

Two units with the same SEER2 can have quite different EER2 values. High SEER2 numbers often come from excellent part-load performance, not from being better at full load in extreme heat. Typical ranges look like this:

These are typical ranges, not guarantees. Read the exact figures for the matched outdoor unit and indoor coil in the AHRI directory or on the manufacturer's product data.

Why EER matters in hot climates

If your summer has many hours above 90 °F, the system spends a lot of time near the EER test condition. Several practical things follow:

• Peak bills: on time-of-use or demand rates, the most expensive kilowatt-hours are late-afternoon ones. A higher EER directly lowers the draw during those hours.
• Grid and backup power: peak watts set the size of a standby generator or battery system. Use the full-load watts above in the generator sizing calculator.
• Southwest minimums: in Arizona, California, Nevada and New Mexico, split ACs must also meet 11.7 EER2 (under 45,000 BTU/h) or 11.2 EER2 (larger), with a lower 9.8 EER2 floor for units at or above 15.2 SEER2.
• Rebates: many utility and efficiency program tiers specify a minimum EER2 alongside SEER2, precisely because peak demand is what they want to reduce.

Real efficiency also drops as it gets hotter. At 105–115 °F outdoors, a condenser rejecting heat into very hot air can lose another 10–20% of its capacity and draw more power than at 95 °F. Shading the outdoor unit with plants (while keeping 2 ft of clearance) and keeping its coil clean helps more in the desert than anywhere else.

Using EER to estimate peak power and cost

Because EER is measured at full load in hot weather, it is the right number for working out what your air conditioner draws on the hottest afternoon of the year.

Peak power (kW) = Capacity (BTU/h) ÷ (EER × 1,000)
Peak cost per hour = Peak power × Rate ($/kWh)

Example: a 4-ton (48,000 BTU/h) system rated 11.5 EER2 draws about 48,000 ÷ 11,500 = 4.17 kW at the rating point. On a time-of-use plan charging $0.40/kWh between 4 and 9 p.m., five hours of continuous running costs about 4.17 × 5 × $0.40 = $8.35. The same unit at 13 EER2 would draw 3.69 kW and cost about $7.38 for the same period, close to a dollar a day through a long, hot summer.

Remember that a correctly sized system only runs flat out on the hottest days. For a month-long estimate across varied weather, the seasonal SEER2 figure is the better input.

Converting EER to COP and kW per ton

Engineers often express the same efficiency in other units. All three describe the same thing:

COP = EER ÷ 3.412
kW per ton = 12 ÷ EER

The COP calculator handles these conversions and lets you work from measured watts and temperatures.

EER and CEER on window and portable units

Room air conditioners are not rated in SEER2. Window and through-wall units carry a CEER (Combined Energy Efficiency Ratio), which is EER with standby and off-mode power folded in. Typical new window units fall between about 10 and 15 CEER, with inverter models at the top. Portable units are rated on a separate SACC basis that accounts for the heat their ducting and casing leak back into the room, so their numbers are lower and not directly comparable to window units. To turn any of these into a running cost, plug the rating into the AC running cost calculator in place of SEER2.

Frequently asked questions