Why the capacity of air conditioners is expressed in tons instead of horsepowers?

Most first-time buyers of air conditioners are confused about why AC units are compared in terms of tons. Why not? We are more used to tons as a unit of measure, implying how much weight a cargo ship or truck can carry. Truth to be told, tons in AC are not related to the weight of the unit. It pertains to how much amount of heat a specific unit can remove from the area in an hour. However, you might still be wondering how tonnage found its way to ACs. Well, the truth traces back in time.

You might be surprised that early people used actual ice for cooling homes and refrigerating their food. That plays a big part in why tons have been used to refer to the capacity of air conditioners in removing heat as it reflects how people used ice before. But today, a five-ton air-conditioning unit is functioning how an actual five-ton ice block would, given that heat is removed evenly, the ice block melter completely, within 24 hours. To put it simply, buying a five-ton unit is like buying a five-ton block of ice that you would use to cool your home for a day, sans all the inconveniences and watery mess.

It may appear that odd why the capacity of air conditioners is calculated in such a way, but the same also happens in other machines or devices that we use today. For instance, let’s look at horsepower in cars. Early people used horse-drawn sleds or carriages before the car was invented. The unit power is expressed today based on the things they have replaced, tons of ice blocks for air conditioners, and the capability of a big horse to pull carriages.

But, how do tons in AC work mathematically? Each ton of ice liquefying steadily in a 24-hour duration can remove heat at around 12,000 British thermal units (BTU) per hour. One (1) BTU is equal to the amount of heat generated when burning one kitchen match.

So, that means that if you get a five-ton air-conditioning unit, it would be able to eliminate 60,000 BTU per hour or remove the heat you get from igniting 60,000 matches. In a day, your AC unit gets to remove a staggering 1,440,000 BTUs. Imagine killing the heat from over a million and a half kitchen matches in 24 hours – your AC unit sure does an incredible job.

But, you might see others still using horsepower when checking ACs. However, it doesn’t refer to the cooling capacity of the unit. Instead, it is used to measure the standard efficiency of the engine, motor, compressor, or the refrigeration unit. The use of HP to refer to AC’s capacity traces back during pre-metric times. Most air conditioners at that time had systems with lower efficiency. With that, it is practicable to use the HP unit instead of the actual cooling capacity. Moreover, AC units weren’t as popular then as today, and energy star ratings weren’t devised yet, getting quite some commercial or sales advantage when using HP instead of the AC unit’s cooling capability.

Nevertheless, we are fortunate that we are using tons to refer to the AC unit’s cooling capacity and not to the actual weight of ice blocks we need to cool our home. We can efficiently and effectively remove heat, without having to carry five-tons of ice to enjoy a cool feeling.

More Readings:

Air conditioner (Wikipedia)

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