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.