A really big one -- 20K tungsten, 24-light tungsten or 12-light with spot globes, 18K or 12K HMI, etc. If you want to look directly at the fixture, you wouldn't want to use a multi-bank light like a Dino, and you'd have to hide the stand (green it out for example, or undersling it from a condor) and hope it flares enough to hide the yoke on the sides of the light, etc. Some smoke on location would help, just like the steam is helping.
If it were a smaller window, you could get away with a smaller light like a 5K tungsten PAR or a 6K HMI PAR. A tungsten lamp would give you a warm color with a gel, which is an issue since you are looking at the lamp itself, if it needs to be gelled, you'd have to gel all the windows.
Your best bet would be to shoot this angle when the real sun is there and then just fake it on other angles.