The single most common shape of question this site gets, and the reason the room cooling calculator exists.
Start with the heat rather than the machine. The calculator asks about the room's size, its glazing, which way it faces and what is above it, and tells you where the heat is coming from before it tells you what size of unit would remove it. In a loft the answer is very often the roof rather than the windows, and no amount of machine fixes a roof.
In rough order of what tends to help most, and what the site has written up:
- Stopping the sun reaching the glass at all, from outside. External shutters and shading beat anything on the inside of the window.
- Insulation above you, which is a winter job that pays in summer too.
- Ventilation at the right time of night, which is later than most people think.
- Then, if the number still says so, a machine sized to the heat gain rather than to the room.
Post your room and what the calculator said, and let us see whether the number matches what the night feels like.