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Start With CEER and Cooling Capacity
FEMP uses the combined energy efficiency ratio, or CEER, in Btu per watt-hour to compare room air conditioners. A higher CEER means the same capacity uses less electricity in FEMP's comparison. CEER explained shows what the measurement means.
The other figure is cooling capacity, measured in Btu per hour for window units. FEMP's worked example uses a 12,000 Btu/h room air conditioner without reverse cycle, with louvered sides, in the 8,000 to 13,999 Btu/h range. For a portable, the Department of Energy capacity figure is the seasonally adjusted cooling capacity, described in BTU and SACC ratings, but FEMP's guidance excludes portable air conditioners, so the worked example below covers window-type units only.
The 750-Hour Example From FEMP
FEMP's table is based on the DOE rating method for a 12,000 Btu/h room air conditioner operated for 750 hours per year. Its figures can be reproduced this way: divide capacity in Btu per hour by CEER, multiply by 750, then convert watt-hours to kilowatt-hours. For a unit with CEER 14.7 and 12,000 Btu/h, the result is 612 kilowatt-hours. For CEER 10.9 the same capacity gives 826 kilowatt-hours, and for CEER 15.4 it gives 584 kilowatt-hours.
CEER is a rated efficiency figure, not a live reading, so the arithmetic reproduces FEMP's published yearly figures rather than the draw at any one moment of operation. Multiplying watts by hours gives watt-hours, and converting to kilowatt-hours gives the unit shown on an electricity bill.
| CEER | Annual energy use for a 12,000 Btu/h unit |
|---|---|
| 15.4 | 584 kWh |
| 14.7 | 612 kWh |
| 10.9 | 826 kWh |
A Worked Comparison at Three Efficiency Levels
FEMP compares a less efficient base model, a model meeting ENERGY STAR requirements, and the best available model. Across the three, annual energy use drops from 826 kilowatt-hours at CEER 10.9 to 612 kilowatt-hours at CEER 14.7 and 584 kilowatt-hours at CEER 15.4. The comparison uses a fixed 12,000 Btu/h capacity and 750 operating hours.
Because energy use is stated in kilowatt-hours, you can apply your own utility rate without relying on any stored estimate. ENERGY STAR room AC criteria shows what certified models must meet under the program.
From Kilowatt-Hours to Your Electricity Bill
Find the rate per kilowatt-hour on your utility bill and multiply the annual kilowatt-hour figure by that rate. The result is the annual energy charge for the air conditioner at 750 hours of operation.
If the unit runs more or fewer hours than 750, scale the number. Fewer hours give proportionally fewer kilowatt-hours, and more hours give more. Choosing the right BTU for your room size helps you match capacity to the space.
Sizing, Sun, and Occupancy Change the Load
Capacity and CEER set the comparison, but real energy use depends on thermostat setting, room heat gain, and run time. ENERGY STAR says a properly sized unit saves energy, while an oversized unit wastes energy and does not provide better cooling. What happens when a room AC is too big goes further.
ENERGY STAR gives capacity adjustments for a heavily shaded room, a very sunny room, extra occupants, and a kitchen installation. These adjustments help match the unit to the room, which ENERGY STAR links to energy efficiency. Choosing the right BTU for your room size explains the full list.
A variable speed compressor can adjust output rather than cycling on and off. ENERGY STAR says many certified models include variable speed technology, and models that meet the program use 9 percent less energy than standard models. Inverter vs fixed-speed room air conditioners covers the difference.
Window Units and Portables: What the Method Covers
FEMP states that its guidance covers certain window and wall-mounted room units, including casement configurations, through-the-wall models, and reverse cycle units. Portable air conditioners are excluded. In the ChillPane catalog, the yearly figures in this article apply to window-type room units. See best window air conditioners.
For a portable, the Department of Energy capacity figure is the seasonally adjusted cooling capacity, not necessarily the box Btu figure. When a portable's CEER is shown, it is tied to that seasonally adjusted figure. Best portable air conditioners shows how the catalog ranks portables.
Which to choose
| If you | Look for |
|---|---|
| If you want a quick yearly estimate for a window AC | look for CEER and listed Btu per hour, then work the 750-hour arithmetic above; FEMP's table itself covers only a 12,000 Btu/h unit. |
| If you compare two models of the same capacity | choose the higher CEER. A 12,000 Btu/h unit at CEER 15.4 uses 584 kWh per 750 hours, while one at 10.9 uses 826 kWh. |
| If you are comparing portable units | compare seasonally adjusted cooling capacity and CEER; the FEMP 750-hour example does not cover portables. |
| If your room is heavily shaded | ENERGY STAR advises reducing the table capacity by 10 percent; a unit sized to the room is the most energy efficient choice. |
| If your room gets strong afternoon sun | ENERGY STAR advises increasing the table capacity by 10 percent for a very sunny room. |
| If more than two people regularly occupy the room | ENERGY STAR advises adding 600 Btu per hour for each additional person. |
| If the unit will be used in a kitchen | ENERGY STAR advises increasing capacity by 4,000 Btu per hour. |
| If you want to know your actual annual energy charge | multiply yearly kilowatt-hours by the rate per kilowatt-hour on your utility bill. |
Questions people ask
Does a higher CEER always mean lower energy use?
Yes, when capacity and run time are the same. FEMP's comparison shows a 12,000 Btu/h unit at CEER 14.7 uses 612 kilowatt-hours per 750 hours, while CEER 10.9 uses 826.
Why does the method use 750 hours a year?
FEMP's published example is based on the DOE rating method for a 12,000 Btu/h room air conditioner operated for 750 hours per year. Your own hours may be higher or lower.
How is running cost estimated without a fixed price?
Multiply the annual kilowatt-hour figure by the rate per kilowatt-hour shown on your electricity bill; the result is the annual energy charge.
Is box BTU appropriate for a portable in this method?
No, and FEMP's guidance behind the 750-hour example excludes portable air conditioners. Federal standards rate a portable's CEER against its seasonally adjusted cooling capacity, so compare portables on that figure rather than the box Btu.
Does an oversized unit use less energy?
No. ENERGY STAR says an oversized unit cools the room before it has a chance to remove humidity, wastes energy and does not provide better cooling; a unit sized to the room is the most energy efficient choice.
What unit is CEER shown in?
Federal efficiency standards state CEER in Btu per watt-hour; for the same 12,000 Btu/h capacity, a unit at CEER 15.4 uses 584 kilowatt-hours per 750 hours, and one at CEER 10.9 uses 826.