APU & FS9

Hello,I'm working/Modifying my Gulfstream G550, I just recently added an APU into the panel. My question is was it a waste of time, or keep it in for more realization purposes? When I do switch it on, I get Power to an already Cold & Dark Cockpit; but I can just turn on the Master Battery, and it does the same thing. Is there more that I can do (or have to do) to the APU to make it more realistic, or is that about it? I'm also looking for a nice looking APU (if there is such a thing) to add/change to my Plane. The one I have I can't remember where I got it from but it looks okay. Not sure if it matters or not but my Plane, and Panel are for my own personal use. Thanks J.R. Duda