I've been doing some research on this topic myself, with the intent of putting a trio of 21" tubes over an ~300G reef we're building into a new house. The intention is for this to be the primary lighting, with just enough supplemental electric light for evening viewing.
If you believe the manufacturer's claims, a 13" tube is equivalent to approximately 100W of flourescent or MH lighting (Sun Pipe claims 300-500W of incandescent, which would be a bit more). A 21" tube should be equivalent to a good 250W MH bulb. The part that's unclear is the conditions where this is true. I would hope this is average for a mid-latitude clear day. If so, the peak noon-time output could be double that.
Looking at it another way, if 50% of the light that hits the dome is delivered over the tank, and if I'm expecting a 21" tube to light 4 square feet, I'll be getting 30% of full sunlight hitting the top of the tank. According to a diagram in Adey's book, the intensity here in Pennsylvania averages about 2/3 of tropical, so I'd be getting about 20% of full tropical sunlight --- maybe more like 25% in June and 15% in December. Even at 15%, that's probably brighter than a 10m deep reef gets in the tropics.
A net search will yield at least half a dozen sources for tubular skylights. For best results you want one that delivers the most light through the tube. The most efficient tubes go straight down with no bends and have very high reflectance on the walls. Since the light will reflect multiple times on the walls, a difference of a few percentage points in reflectance can have a dramatic effect. Some have an acrylic dome, which is less likely to yellow than other plastics. Some also offer a reflective collar that dramatically increases the light when the sun is at a low angle (e.g., in December). I'm planning to use
Sun Pipe, which seems to have all the right features.
I have a quote to install a pair of 13" Sun Pipe tubes for $680 to go over a refugium. That's new construction, so I would expect an add-on to be more. The advantage of a 13" tube is it fits neatly between your rafters. I have no idea what my 21" tubes will cost... that's buried somewhere in the cost of the house. Assuming each 13" tube saves annually about $50 in electricity and $50 in fancy light bulbs, the 13" tubes will pay for themselves in about 3-4 years.
