Plants undergo three shocks when switched from inside to outside: Light, temperature and wind. Add ripping the pot from under them, and they can be set back a week or two of growth. Using a cold frame to protect the potted plants from evening cold and morning frost while they become accustomed to the wind and partially-deflected sun is a good call, but not necessary if its after last frost. After all, they'll be set back for a bit either way.

I have planted out directly from the grow lights and from 'hardened off' plants, and they both seem to take around 10 days before growing actively again. If the ground is cooler, it takes longer. With window-grown plants, you might find that the stem withers up and a new, much stronger, sprout comes up next to the first ailing sprout that had inadequate light. It's all good, as having a decent root system going into the ground is more important then a taller sprout.

What type of grow lights are you running? I use seven 4 foot shop lights, and they don't seem to cause too much drain. They aren't as good as the 'real deal' grow bulbs, but work well enough for me.

I don't know much about them: I have two large round grow lights that my husband had from his college days (I didn't want to ask why he had these). They have a central mechanical part and a large bulb that is about 6 inches long that hangs down below the light. On top there is a round silver reflector to keep the light shining downward. They put out a huge amount of light, and a decent amount of heat. I have them strung up from the floor joists in a large walk-in closet I have in the basement (8x10ft). I only put some of my plants in there because I had no idea what to expect - I didn't know if the lights would fry them or what would happen. I have the plants, in pots, sitting on the floor, and the lights are about 4 feet high. When the lights are on, it looks like an alien invasion is going on in there! In any case, the plants are thriving. I'm pretty much just "trying it out" this year. If they survive the transplant shock, I will set up a more serious system next year. My dahlia habit has grown the past few years to become a bit of an obsession - They are literally taking over my house (and garden!).

Sweet! Definitely would explain your crazy electric bill, but if you have them in an 8x10 closet, you could put a lot of plants in there! If it was a smaller space, I'd warn you about ventilation with those babies, but it sounds like it'll work for this year as-is. My 4 foot fluorescents don't give NEAR the output as yours do... of course, they don't put as much of a drain on the electric, either.

"become a bit of an obsession"
Ha HA! I'm with you, Lady! Keep on plant'n...

I used to throw them in the garden as an afterthought - now they have their OWN garden. And apparently their own little growing spa room. With an attendant (me). If I do decide to expand next year, I thought I'd put them on plastic trays. I can leave one of the doors open (The closet has two of those bifold doors) Do I need to set up a fan? Its colder in the basement than in the closet (the light gives off a lot of heat). Leaving the door open also means that the kids might get a suntan when they are down there playing. Not sure that's such a great idea! Will have to do more research before then...