EER.
Efficiency measured at one hot temperature. For this city, more honest than the seasonal number.
EER is the Energy Efficiency Ratio: cooling output in BTU per hour divided by electrical input in watts, measured at a single set of conditions, traditionally 95 degrees outdoor.
SEER2 is the seasonal version, averaged across a modelled range of temperatures including mild ones.
Why EER suits Phoenix
A seasonal average is useful where summers are moderate and the system spends much of its time at part load. Phoenix is not that place.
Here the system spends months at or near the hot end of the range, which is exactly where EER is measured and exactly what a seasonal average dilutes. Two systems with the same SEER2 can perform noticeably differently at 110, and EER is the number that hints at which.
If you are comparing equipment for a Valley house, EER deserves at least as much attention as the headline seasonal figure.
What both numbers leave out
Everything about your actual house.
- Duct leakage, which is frequently the single largest loss
- Whether the system is correctly sized
- Refrigerant charge being right
- A condenser coil clean enough to reject heat
- Insulation and what your roof does at 115
A high rated system with a fouled coil and leaky ductwork will lose to a modest one that is clean and correctly charged. The rating describes the equipment in a laboratory, not the machine bolted to your house.
What you can actually change
On equipment you already own, the rating is fixed and the three things that move your running cost are not: charge, airflow, and coil condition. All three are measurable, and a check-up writes all three down.
Nobody here sells equipment, so this page has no interest in which number you buy.