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I want to research into electrical consumption of air conditioning units. While trying to make a rough estimate of efficiency, found that the btu/h capacity of the specifications does not make sense with the electrical consumption of the unit. For example, one unit reports 12000 btu/h and 950 W of peak energy consumption, but one converts 12000 btu/h and obtains 3516 W of cooling capacity so something feels off.

I want to delve the less I can into the EER or SEER efficiency ratings because I suspect there will be lots of arbitrary decisions or references that will not be helpful enough. Is there a way to know the absolute efficiency of the A/C units (in terms of heat transferred relative to energy usage)?

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  • You say you want to research this, but also that you want to avoid delving into EER or SEER. These metrics take into account the impact of ambient temperature on a/c performance, so they're hard to avoid. What's the question you're trying to solve?
    – LShaver
    Commented Jun 27, 2023 at 23:39
  • Have you looked into other alternatives of cooling, such as insulation, reduction of incoming solar radiation, night cycling, geothermal cooling, and others? A classical air conditioning unit can use huge amounts of energy, as a brief glance at the ludicrously high summer domestic electricity consumption in the USA shows (my German household uses per year what an average Texas household uses per month).
    – gerrit
    Commented Jun 28, 2023 at 7:34
  • Using your numbers, 950 W of electrical energy is used by the AC to produce 3516 W of cooling energy.
    – Fred
    Commented Jul 13, 2023 at 8:54

1 Answer 1

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The efficiency of an air conditioning unit is a simple division: divide heat extracted from the cold reservoir by the amount of work it takes to extract that heat.

Note that an air conditioning unit is a heat pump. It's the opposite of heat engine. A heat engine moves heat from hot to cold, while generating useful work from that. The useful work will be less than the amount of heat it takes from the hot reservoir.

A heat pump moves heat from cold to hot, while doing work and requiring an external energy source to do that work. The work done will be less than the amount of heat it puts to the hot reservoir. In many cases, it's also the case that the work done will be less than the amount of heat it takes from the cold reservoir.

The efficiency of either heat engine or heat pump can never exceed the Carnot efficiency. The Carnot efficiencies are the inverses of each other. This explains why a heat pump can have a coefficient of performance of greater than one: a heat engine always has an efficiency of less than one.

If a heat pump has a coefficient of performance of greater than two, it has an energy efficiency rating of greater than one. EER is simply COP - 1.

COP is useful when heat pump is used to create heat, EER is useful when heat pump is used to cool down something.

If it has 3516 W of cooling capacity for 950 W of heat input, it means it has an EER of 3.7. Perfectly reasonable with today's technology, although this EER is only valid for the measurement conditions: a certain indoor temperature and a certain outdoor temperature.

If you want to save energy, what you want is:

  • A heat pump with EER as big as possible.
  • Preferably a split unit as opposed to a unit with evaporator and condenser in the same unit
  • If you really have to use a ducted single-unit non-split system, prefer systems with two ducts as opposed to systems that take the air from indoors and blow it hotter to outdoors, since all the air blown outdoors will be replaced by outdoor air, and if it's so hot outdoors that you need air conditioning, you will get a lot of hot outdoor air as replacement air
  • If the heat pump creates so much sound indoors that it's annoying, you might be less annoyed by a continuously-running quieter sound than an intermittent louder sound. So a variable inverter drive for the heat pump is useful. The most expensive units, especially the permanently installed split air conditioners, usually have an inverter drive. Portable units often lack inverter drive.

Note also that some definitions of EER perform the division using different units at the dividend and the divisor. This creates a strange number (with units of BTU/(watt-hour)). You shouldn't use such strange numbers, you should divide only watts by watts. But if you see strange-looking EER values, it may be the case that the division uses inconsistent units. Don't contribute to the proliferation of such inconsistent units!

Also since we are here interested in sustainability, you might want to consider cooling down your house during night and then turning off the air conditioner during day. This is for several reasons:

  1. Energy efficiency of air conditioners is better when it's hotter indoors and colder outdoors. So during the night, you get the most amount of heat moved for the same amount of electricity used.
  2. Electricity is cheaper and more sustainable during the night since there are less electricity users but usually nearly the same amount of electricity generation (with the exception that you obviously lose solar power at night). Fossil fuels are more likely to be burned during daytime.

Obviously this strategy requires you having a big enough air conditioner, and it also requires you to tolerate a coolish house during the morning, with temperatures increasing towards the night when the air conditioner is again turned on.

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