I have a light fixture with 9 G4 20W halogen 12V light bulbs. It has Jindel 105W 120V to 12V transformer, which died. Please explain how 105W transformer can support 9 20W light bulbs? Should it be 180W (9*20W=180W) transformer? I couldn't find similar transformer with 180W, but I found 150W. Will it work in my fixture? Again, why OEM installed 105W transformer? Thank you in advance for the response!
I think you are asking the right question. The transformer should be rated higher than the total load. It would be better to go higher with a landscaping type transformer this one is 12V @ 500W. You don't want to use a switching supply as they have a tough time with loads like halogen lights.
Alternatively, consider swapping out your surface-of-the-sun hot halogens for some LED replacements and going with the lighter transformer you found. Make sure to select a dimmable product if you need that feature.
Please explain how 105W transformer can support 9 20W light bulbs?
The 105 watt rating is for continuous duty in nominal conditions for a specified temperature rise.
No doubt in your situation it was getting much warmer than specified and that is certainly why it died. How long it lives depends mostly on the temperature the transformer reaches. If it is installed in a cool, well ventilated place, and runs at most an hour at a stretch, it might last for years.
If you want a long-lasting installation, significantly oversize its replacement, like 400 watts. Or, better yet, replace the halogens with LEDs.
why OEM installed 105W transformer?
Maybe there wasn't 180 watts initially? Maybe there were 12 watt bulbs originally 9x12 = 108 watts—close enough considering the 10 or 20% variability of the components. Or maybe there was a dimmer. Or, maybe the OEM did not know what they were doing...