First enable the AE Lock.  Place the primary light (daytime windows) to your back then rotate 90 ° left or right.  The first image in the series the Automatic exposure lock will acquire the exposure settings.  So I find that putting 30% of the window.... as well as the low level light of the room will record an "Average  aperture/ speed" settings on the 1st shot of the panorama image series.  The camera settings will be reused by the camera for the other shots.  

After the last shot.. then RESET or turn off the AE lock.  The next room will have its own lighting conditions.

Dave