What exactly is the difference between two ACs with different SEER ratings?

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Some companies sell similar looking models, with same cooling capacity, but difference in energy consumption. Which components within similarly-spec’d ACs contribute to energy efficiency, exactly?

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How efficiently the motor produces rotation from electric power. How much of that input shaft power is consumed by friction inside the compressor. How much power is used by systems that don’t directly cool the refrigerant, circuits, relays, fans, … .

Most of it comes down to the motor in the air handler adjusting itself to various speeds depending on the cooling needed to run. Normal PSC motors are 100% or 0%. ECM motors can be set to different levels of speed: high medium or low basically. High end motors can ramp up or down with variable speeds depending on the amount of air flow needed to cool your home. They generally can run from 40%-100%.

In addition to that you also have larger physical evap and condenser coils so they can transfer heat easier because they have a larger surface area.

There are other ways like 2 stage compressors as well but its mostly the fan and coil size between 13 and 16 SEER

There are other ways, like inverters but those are on very high SEER and minisplits

The amount of savings you get will vary wildly but if you don’t have good windows, doors, duct and insulation a higher SEER rating will do very little for you.