Good Luck for your growth. There are not any peculiar grow light, you can use a regular LED light in blue for vegging and red for flower to achieve same results.

Plants can use the whole spectrum, but they use red most efficiently and they need some blue to avoid getting leggy and so on. If LED grow lights live up to their labeling, they theoretically should give the lowest possible electric bill while still providing adequate light to your plants. But that has to be weighed against the up-front cost.

That's in theory. I don't have the practical experience to say much.

A 20W incandescent doesn't use much electricity, but it also doesn't put out much light. A 13W CFL from the dollar store would give more, and it's so cheap it would probably pay for itself before it burns out, even off that dinky 7W savings. A 5W LED should give more light than the incandescent, at least. But even with LED efficiency, 5W isn't much.

How do I know how much light a plants needs (not in terms of hours)? For example, what can I grow with a 240-lumen LED?

Hello, I have successfully grown a dwarf tomato variety (Red Robin) under a combination of cool white and warm white leds in doors this winter. The combination used is two warm white to one cool white. That is the same combination you would use with fluorescent lights. I did not use any far red or low blue leds in this setup. I use roughly 50 watts to a square foot giving me approximately 4,000 lumens. You can use only the cool white leds to get seedlings started but no lass than 3,200 lumins to a foot. I have had problems with tomatoes and pepper plants growing too long and falling over with less lumins per foot. Hope this helps.