I am trying to replace the air conditioning device at home but I am not sure how to compare my old Panasonic that was bought in 1999 (possibly model of 1997 or 1998). Some facts:

  • I was not able to find details of the current device online. There is no archive of old systems.
  • It is a 18000 BTU machine
  • Main reason for using is cooling

The main reason for replacing it is the energy consumption. I do not have any metrics for efficiency to compare with the aircons today.

My main question is that shouldn't the new aircons of 12000 BTU be more efficient that the old ones of 18000 BTU? After 24 years, I would expect some kind of a similar logic to apply.

  • 3
    Find and take a clear picture of the labelling on your unit - it may reveal things you don't know it's telling you. edit to add that picture.
    – Ecnerwal
    May 25, 2022 at 12:08
  • Is this a window unit or outdoor unit?
    – MonkeyZeus
    May 25, 2022 at 14:02
  • Air condenser efficiency comes in 3 main flavors: single-stage, two-stage, and variable speed compressors. Yes, there have also been advancements in lubrication of components so there's less resistance and less energy wasted. Unless you've greatly improved your home's thermal barrier or trees are providing lots more shade do not downgrade to a 12000 BTU because it will need to run longer and will cost more to achieve the same cooling as the 18000. You also need to consider coil surface area. This is usually too technical for the average homeowner so just check the SEER rating.
    – MonkeyZeus
    May 25, 2022 at 14:05
  • You should probably also consider a heat pump, rather than a standard air conditioner, as heat pumps can be substantially more efficient. However, what will work well for you will depend on your specific application, the models which are available to you in your area, and depends on multiple factors which aren't included in this question.
    – Makyen
    May 25, 2022 at 20:06

4 Answers 4


BTUs are used to size the unit to area of space to be cooled.

What you are thinking of is the electrical efficiently of a unit, which a new 18000 unit today should use less electricity than your old unit for the same size of area to be cooled.

Find the area to be cooled, then find find how many BTUs you need. Over sizing or under sizing the BTUs is bad for efficiently.

  • Have the measurements/standards for BTUs stayed the same over the years though? I have a portable AC that claims 12k BTU, but then in the small print states that its 10k BTU according to the DOE standard. I could imagine stricter standards over the years meaning that a 24 year old "18k BTU" unit would be today sold as a smaller number.
    – mbrig
    May 25, 2022 at 16:47
  • 1
    BTU units have not changed since developed(~100 plus years or more). Marketing speak is different and might fudge terms. A 10k unit as good as a 12k unit in a well insulated room, is still a 10k unit. Well insulated walls and ceiling/roof would do wonders for efficiently, and a 10k unit might be good enough, but poor/not as well insulated will need a 12k unit in the same size of room.
    – crip659
    May 25, 2022 at 17:03
  • 2
    @crip659 But BTU rating will usually vary depending on temperature difference between indoor and outdoor, and there are standards that define the conditions used for the rating printed on box. Without standards everyone would rate their device as "20000 BTU*" "* Only when below 20°C outdoors". Common standards are listed e.g. here: learnmetrics.com/btu-sacc-vs-ashrae
    – jpa
    May 25, 2022 at 17:05
  • @mbrig Your particular unit has a dual rating because it's a one-hose portable A/C, which is a dementedly horrible design. It's nearly not your fault, since 2-hose units are hard to find, because consumers think 1 > 2. The lower rating tries to account for 1-hose units' self-sabotage. May 25, 2022 at 21:11
  • @Harper-ReinstateUkraine its actually is a dual hose model. I'm not sure where the penalty comes from in that case... maybe they just labelled something without thinking?
    – mbrig
    May 25, 2022 at 21:19

Is the 18,000 BTU/hr unit oversized for the area being cooled?

If so, downsize; if not, stay at the same cooling capacity.

A 12,000 BTU/hr unit of the same era/efficiency would use 2/3rds the power that a 18,000 BTU/hr unit of the same era/efficiency would, while cooling only 2/3rds as much.

In 24 years, there have been advances in efficiency as well. so a modern 18,000 BTU/hr unit will (generally, shopping still applies as inefficient crap is still made) use less power than an 18,000 BTU/hr unit from 24 years ago while providing the same amount of cooling. You can measure the input power to your old unit and get a rough idea if you can't find the SEER (or just EER) ratings on it.

The label on an older air conditioner here lists the date it was made, refrigerant type, cooling BTU/Hr, cooling amps, cooling watts, and EER (which is BTU/hr / cooling watts) - that's the Energy Efficiency Ratio, which can be compared with a new unit. Bigger is better for the EER number. Most or all of that information should be on the label of a late-1990's unit as well.

If you'd like the best possible efficiency (and a much quieter unit, generally) albeit at a higher initial cost, look into a modern Mini-Split unit, as opposed to a window unit. You get your window back, and the noisy parts are relocated outside the house. The higher initial cost is offset by the lower running cost (you might also be able to get a rebate from your power company.)

Improving your house's insulation will help with the running cost of any A/C or heating.


There are hard numbers which you can judge a unit by:

  • BTU rating, which in HVAC equipment means "BTUs per hour". A BTU is still the energy needed to heat 1 pound of water 1 degree F.
  • The machine's rated voltage
  • The machine's rated running amperage on its nameplate; however, this indicates the circuit capacity needed to power it, not the true usage.

The last two multiply together to get watts, divide to get a rough estimate at BTUs per watt. BTUs per watt is also called "SEER" for air conditioners (and heat pumps in A/C mode).

Nowadays most units state the SEER right on their labeling, and this number is based on precise lab testing of actual draw.

You can get any efficiency you want to pay for, as high as 38 SEER. However in the highest SEER ratings, you may find yourself in heat pumps, which means cheap heat too.



Ultimately, one BTU refers to the amount of energy that’s required to increase the temperature of a pound of water by 1° F. In the case of air conditioners, it's a measure of how much heat they're capable of removing from a space.

BTUs refer to the actual thermal transfer though, and not to the input energy, so the numbers remain constant. As others have said, if an old 18,000 BTU unit is adequate for your space, do not downsize to a 12,000 BTU unit (you could try a 15,000 or 16,000 BTU unit but those would have to work harder). Instead, a modern 18,000 BTU unit will provide the same amount of cooling as your old unit*, but consume less electricity to do so.

* technically a new unit will provide as much cooling capacity as your old unit did when it was new... coolant losses, dirt on coils and fins, etc will have reduced its cooling capacity over the years, though to some extent those can be compensated for -- washing the fins/coils, recharging coolant, and so on.

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