Good question, it has been very hot during our trip, especially in Kyoto but also Tokyo and Nikko were hot. We like our room to be 21°C when we sleep and 23°C during the day. For our room (107) this worked well. The room was too warm when we arrived and it took 2 hours to cool down to the correct temprerature’ For our childrens’ room (106) we did not manage to get the temperature below 24°C, their hot tub seemed warmer and as they were less strict on keeping the doors between their living room and bathroom closed we assumed that this had an influence, the hot tubs are very large and create a lot of heat.
The a/c can be set lower (as low as 16 if I remember correctly) but I’m not sure if it would effectively be able to cool the room down that much.