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I'm not sure bobflux's answer is correct. We're not interested in cooling to thermal equilibrium: we want to cool the air in the garage to 18 C, and then maintain that temperature by balancing the rate at which heat enters the room from the walls with the rate of cooling input. What matters is that you feel the room is 18 C, not that you cool the garage in totality to 18 C. In that sense, this happens all the time with domestic air conditioners, like a NYC apartment with a window unit, since as soon as you turn the unit off the temperature rises again. I think this is a roundabout way of saying, just size the unit for the space and go workout.

I'm not sure bobflux's answer is correct. We're not interested in cooling to thermal equilibrium: we want to cool the air in the garage to 18 C, and then maintain that temperature by balancing the rate at which heat enters the room from the walls with the rate of cooling input. What matters is that you feel the room is 18 C, not that you cool the garage in totality to 18 C. In that sense, this happens all the time with domestic air conditioners, like a NYC apartment with a window unit, since as soon as you turn the unit off the temperature rises again. I think this is a roundabout way of saying, just size the unit for the space and go workout.

Edit: To add more details, you would calculate the rate of heat transfer from AC to air, and room to air, then you would solve for the fixed rate of cooling from the AC that leads to a stable temperature at 18 C. In reality, the AC probably has a control system that varies the amount of cooling based upon distance from the set point. It will surely output at max BTU when you first turn it on, then as it approaches 18 C it will dial its BTU back to the point where the cooling it outputs is balanced by the room. So, in my opinion, the entire problem is taken care of by the AC, and under or oversizing is moot if you only intend to use it transiently. Sizing an AC is important for continuous operation.

I'm not sure bobflux's answer is correct. We're not interested in cooling to thermal equilibrium: we want to cool the air in the garage to 18 C, and then maintain that temperature by balancing the rate at which heat enters the room from the walls with the rate of cooling input. What matters is that you feel the room is 18 C, not that you cool the garage in totality to 18 C. In that sense, this happens all the time with domestic air conditioners, like a NYC apartment with a window unit, since as soon as turn the unit off the temperature begins to rapidly rise again. I think this is a roundabout way of saying, just size the unit for the space and go workout.

I'm not sure bobflux's answer is correct. We're not interested in cooling to thermal equilibrium: we want to cool the air in the garage to 18 C, and then maintain that temperature by balancing the rate at which heat enters the room from the walls with the rate of cooling input. What matters is that you feel the room is 18 C, not that you cool the garage in totality to 18 C. In that sense, this happens all the time with domestic air conditioners, like a NYC apartment with a window unit, since as soon as you turn the unit off the temperature rises again. I think this is a roundabout way of saying, just size the unit for the space and go workout.

I'm not sure bobflux's answer is correct. We're not interested in cooling to thermal equilibrium: we want to cool the air in the garage to 18 C, and then maintain that temperature by balancing the rate at which heat leaves from the room into the walls with the rate of cooling input. What matters is that you feel the room is 18 C, not that you cool the garage in totality to 18 C. In that sense, this happens all the time with domestic air conditioners, like a NYC apartment with a window unit, since as soon as turn the unit off the temperature begins to rapidly rise again. I think this is a roundabout way of saying, just size the unit for the space and go workout.

I'm not sure bobflux's answer is correct. We're not interested in cooling to thermal equilibrium: we want to cool the air in the garage to 18 C, and then maintain that temperature by balancing the rate at which heat enters the room from the walls with the rate of cooling input. What matters is that you feel the room is 18 C, not that you cool the garage in totality to 18 C. In that sense, this happens all the time with domestic air conditioners, like a NYC apartment with a window unit, since as soon as turn the unit off the temperature begins to rapidly rise again. I think this is a roundabout way of saying, just size the unit for the space and go workout.