All windows should have glass energy performance ratings. If the manufacturer won't furnish you with the actual ratings, then most likely the glass is not the high performance glass that should be used in sunroom applications. There are many different types of Low- E coatings. The term "high performance" can be misleading but not untruthful, for example ordinary house windows with a Low-E coating can be called a high performance window, especially when compared with clear glass, single pane house windows. But it would not be a good choice for a comfortable year-round use sunroom.
Several factors should be considered for glass roof rooms. You want the best combination of glass that will give you the greatest efficiency in comparison to visibility. There are some glasses that have good energy ratings but to get the good rating, have a very dark and/or highly reflective tint. These tints are so dark that there is basically no nighttime visibility. This contradicts one of the main reasons for a glass roof room, to be able to stargaze and enjoy the beautiful nighttime views.
There is a difference in glass ratings and window ratings. When using windows ask if the windows are NFRC (National Fenestration Rating Council) certified. Ask to see the certification label (it should be attached to each window). This label will show the certified energy performance ratings. If it is not certified, don't buy it.
AAMA (American Architectural Manufacturers Association) certification grades the structural performance of the window; air infiltration, water infiltration, acoustical and security. This certification is important but has nothing to do with the glass energy performance of the window.