They've now actually reached the top of the window

Impressive. That's allot of growth from solo-cups.

To answer your question, around 4,000 lumens at a 5k color temp.

Extended answer: You're going to want to cut them back, up-pot them and plan for poor growth. Now i say all this, but seeing just how well you've done with just solo-cups by a window, you could possibly get a harvest over the winter if you improve their growing conditions.

I'm with Robert, in that I don't think they will do very well under a grow lamp because they are so tall. If you want to grow tomatoes under lights, you would do better to grow some micro tomatoes that stay nice and short.

Linda

Do you use the radiator? Having your plants laying on a heat source like that might be more of a problem than lack of light is.

Youre going to need 6500k for leaf growth and 2700k for flowering. You’d need to hand pollinate.

even in the very best situation, you’ll be lucky if you get any useable fruit inside. Cherry tomatos will be easier than larger ones. As in, larger tomatoes under lights likely won’t produce.

You'd probably have good success with a few cfl's at 5000k to 6500k. Alternatively, if you have don't mind the blue / purple lights or can run them when you're not in, I've had great success with Phlizon lights and they're relatively inexpensive. If you go with the later option of the two just slowly start, maybe 15 min, double use every day until it's running as long as you want.

Good Luck, a video with some more information posted below.

Some edible tomatoes even in winter

That’s a lot of work and a lot of plant for just one tomato.