Oooh, awesome to know that you can fruit them in such small pots and such a small size
A couple fruits every now and then is all I need from dragonfruit. I like the fruit because they're so beautiful; the taste is kind of bland, IMHO. As for varieties, living in Iceland, I'm kind of stuck with the variety I have at least for now, heh
It's not that you can't get unusual plants here which are not adapted for the climate (I picked up my olive tree here for example). It's just that the safe default assumption here for tropical non-houseplants is always "not available until proven otherwise". I forget which variety I'm growing, it's been so long since I ordered it, but I did compare a couple varieties before selecting this one.
As for the light, well, you have to compare it to the sun. Most of the amazon basin gets 3-5 kWh/m˛/day. So let's say 4. That's an average of 167W. The peak solar intensity on a perpendicular plate on the surface is 1000W, but there's night, there's clouds, there's angles, etc.
Now we look at an artificial lighting setup. First efficiency comes into the picture - not luminous efficiency, but quantum efficiency (luminous efficiency takes into account how the human eye sees light, which is irrelevant for plants; the human eye sees green best while green is worst for photosynthesis). You'll get up to 30% on a good tube. LEDs can approach 40% external quantum efficiency. Because of bulb age and not having optimal bulbs to begin with, let's say 25% and 33% respectively. Then there's light loss. On Earth, the sun is always striking *something*, and if you have a plot of plants, the light that escapes hitting your plants is offset by light that escapes hitting other plants and hits yours instead. You set up grow lights indoors, part of the light will escape and hit... well, something you don't care about. Let's say 60% is useful. So right there are some pretty massive losses. Now there's the spectrum to take into account. Actually, this is an advantage for indoor growers, because a third of the sun's energy is in infrared, which is worthless, and the rest pretty evenly spread, while photosynthesis absorption bands aren't. Fluroescents are sort of a mixed bag in terms of what visible spectrum they provide, but at least there's not much IR** -- let's say credit them with 120% of the sun in terms of spectrum efficiency. LEDs, with blue and red, esp red, are spot on for photosynthesis; credit them with 200% spectral efficiency. Note that we're not considering hormonal effects of a different spectrum than the sun; for example, while my dragonfruit loves all that LED energy, I know from experience that if I were to try to grow, say, lettuce under it, the lettuce would be horribly lanky and basically a disaster. But here we're just talking about energy.
So, to match the Amazonian sun with fluorescents, you need 927W/m˛. With LEDs, it's 421W/m˛. Note that that's per square meter. Growing even a couple square meters indoors like I do, and well... matching the sun takes a lot of light! During the winter I'll have to replace pretty much the entire sun; there's several hours of "dim" outside but the sun won't rise over the houses to my south until late February or so. During the summer at least I have those nice picture windows to help out.
I really think that the number one limitation of most people growing plants indoors is not grasping how much energy plants really get from the sun and how little most indoor lighting provides. Our eyes see light logarithmically, not linearly. It may not look that much brighter outside, but it really is. If we saw light linearly, outdoors would be blinding and indoors would be like a cave. And it makes sense that it takes so much energy to produce food when you look at internal plant losses; food crops which are fully healthy and getting enough light (and everything else) range from a tiny fraction of a percent to 1% of their energy going into the final edible portion (fruits and vegetables), to a couple percent (grains), to as much as 10% in extreme cases like sugarcane. So a 1% efficient plant in the Amazonian sun is actually only getting around "productive" 1 1/2 kilowatt hours of energy per square meter month! That's 1200 food calories per square meter per month. And if you "underfeed" the plant in terms of light, it's going to be needing all of the energy it gets just to try to stay alive.
** To be fair, there actually *is* a lot of IR, but it's far IR, heat radiating from the glass and ballast due to the efficiency losses - and since that's already counted under the efficiency losses, there's no need to count it twice.