Today was the first warm day of the season here in Sunnyvale, CA. In my living room, the air temperature was close to 90*F
The aquarium temperature was a steady 80*F throughout the day.
The temperature controller claimed to be using only 10-15% of the chiller's capacity, but I think this might be misleading because of the way the system is setup. The chiller (water cooler) has its own thermostat, and attempts to keep its water temperature around 45*F. The fishtank's temperature controller turns a pump on and off that pushes the chilled water through a heat exchanger with the aquarium water. The amount of time that the pump is running is the "percentage of capacity" that I have been listing here and in other posts. I am sure the system has a non-linear response such that the amount of cooling delivered at %100 would not be ten times the cooling delivered at %10. This is because the chiller's water temperature would be rising (making it less effective at cooling the tank) as the pump runs more often. Nonetheless, I think the chiller system has plenty of headroom, and it only rarely gets hotter than 90*F in my house, so I am feeling pretty good about the project.