With fluorescent lights, you can run the lamps at partial output. For example, a common battery pack puts out 300 lumens from a 4' 32 watt tube instead of the standard 3000 lumens. That saves on battery size and life.
On the other hand, if you want to run a 400W MH lamp off a battery, you have to supply the full 400W. In general, you need 90 minutes of emergency power. So it would be hard to get 400W for 90 minutes without a large expensive battery.
Steve