By using the same system as power back up generators. They work when the power is cut they start up and run. Same idea just change a few things and you use a timer to start up the generator then use a second timer to start your lights a few minuets after the generator is up and running and turn off once the lights are out.

Digital ballast do not use as much electricity as the current ones do. You may want to look into those. I think 600w is the largest they offer now but they are working on a 1000w version. I know the owner of Advanced is working on one transformer to power up 4000w of lights too and that will cut all out energy cost. I'm not sure if they are on the market yet but they should be soon.