Air conditioner ratings for efficiency

Are ENERGY STAR units always the most efficient?

Most websites on air conditioning ratings will recommend you buy an ENERGY STAR rated air conditioner. These energy saving air conditioners will undoubtedly save you money compared to the cheapest, least efficient on the market (since the ENERGY STAR ones will be at least 10% more efficient).

ENERGY STAR air conditioners will definitely be more efficient than anything more than 8 or 10 years old, because as mentioned in my overview article Energy saving air conditioners, efficiency requirements keep rising, and because old air conditioners can become less efficient with time.

But there can be a range of efficiencies within ENERGY STAR air conditioning ratings.

For example, in the 5,000 to 5,999 BTU range, all room air conditioners currently manufactured need to have an Energy Efficiency Ratio (EER) of 9.7, while an ENERGY STAR unit needs to be at least 10% more efficient, with an EER of 10.7. But if you look at the air conditioning ratings you will find that there are models with an EER of up to 11.2, or 15% higher.

In the 6,000 to 8,000 BTU range there are models up to 24% more efficient than the minimum standard, while in the 8,000 to 20,000 range you can get up to 36% higher than the minimum (or 26% higher than the least efficient ENERGY STAR model in that range of the air conditioning ratings).

Pay close attention to the EER air conditioning rating for each air conditioner. If you visit the ENERGY STAR website, which provides air conditioning ratings for any ENERGY STAR qualifying air conditioner, this is the most important number.

Even with two units that have the same BTU setting, the minimum EER requirement for the unit to be available for sale in the US may vary, depending on certain features the unit has such as louvered sides or whether the unit is window-mounted or goes through a wall.

For example, consider these two units:

As you can see, the Fedders unit appears to be 11% better, while the Carrier unit is only 10% better. But in fact, the Carrier unit has a much higher EER of 10.8 versus the Fedders rating of 9.4, so the Carrier is actually 15% more efficient than the Fedders unit, while appearing 1% less efficient.

In other words, the EER rating shows that the Carrier unit has a considerably higher efficiency even though both are ENERGY STAR certified (and in each case, they just barely manage to beat the standard by the minimum 10% required for an ENERGY STAR designation).

Remember. Don’t take a salesperson’s word on energy efficiency – they’re in the business of selling what they have to sell! That means:

1. Insist on seeing the government-approved energy efficiency tag on any air conditioner you are thinking of buying, and don’t take the sales person’s word that “It’s very efficient” since this doesn’t tell you what you need to know.
2. Don’t buy from an agent who tries to tell you that buying an energy saving air conditioner is a waste of money. Between a window unit costing $200 that is at the EER standard, and another costing $250 that is 12% better than the standard and is therefore ENERGY STAR designated, you will save the difference in cost in just eight years assuming you pay $0.10 per kilowatt hour.

Do your own research online and find out how close the models you’re considering for purchase are from the highest possible EER rating. Given how much electricity even a 5,000 BTU room air conditioner can consume in one cooling season, you want to get the most energy saving air conditioner you can.

And remember that even though the large majority of the air conditioners you will see have an EER rating of not much higher than the Federal standard (between 8.0 and 9.8 depending on the capacity and the age of the unit), there are air conditioning systems out there with EER ratings as high as 14. While they are hard to find, if you use AC a lot they will pay for themselves quickly.

SEER and EER ratings

Air conditioner ratings are sometimes provided in a Seasonal Energy Efficiency Rating (SEER) rather than the standard EER (Energy Efficiency Rating). Here’s the difference:

• EER rates the energy efficiency of an air conditioning unit by looking at the ratio of output cooling (in BTU/hour) to the input power in Watts at a given point in time (e.g. for a one-hour period) and for a given temperature difference between inside and outside
• SEER measures the ratio over an entire cooling season, and considers a range of temperature differences assuming seasonal fluctuations in outdoor temperature.

Since the higher the difference between indoor and outdoor temperatures, the less efficient an air conditioner tends to be, SEER is a more accurate reflection of typical energy efficiency of an air conditioner through a cooling season.

While there is not an exact conversion formula to go from EER to SEER or vice versa, the following are good rules of thumb:

EER = SEER x 0.83    SEER = EER x 1.2

However, the conversion rate for EER to SEER can range anywhere from 0.69 to 0.93 depending on the unit.

Note that the formulas given here differ from what you see on other sites. For example, the Wikipedia entry for Seasonal energy efficiency ratio suggests that EER = SEER x 0.875.

I derived my formula of 0.83 by looking at 2500 air conditioners of various capacities and efficiency ratings, computing the ratio of SEER / EER for each one, and choosing the most common ratio. 65% of the air conditioners rated had a ratio between 0.81 and 0.84, while only 12% had a ratio of 0.875 or higher.

This suggests that the Wikipedia ratio of 0.875 is a bit high.

Doing your own research

To find more information on air conditioner ratings for ENERGY STAR qualified air conditioners, see:

• ENERGY STAR: Room air conditioners
• ENERGY STAR: Central air conditioners

The Room air conditioners section on the ENERGY STAR website includes a link to a spreadsheet showing the efficiency of all ENERGY STAR qualified air conditioners. Unfortunately, the central air conditioners page on the ENERGY STAR site only provides a link to a manufacturers’ association website that contains a virtually unusable database of air conditioners.