Optimal UVA/UVB dosage/bulb/fixture for bunnies in indoor hutches during heat waves

We have three bunnies in indoor hutches. It is dark in the room. Usually I put them in outdoor cages half the day every day most of the year. For the next two months, it is too hot (90-110) so am having to keep them indoors. Am seeking floor lamps that will hold uva/uvb bulbs but don’t know what dosage they need. I have read that the lamp should be 9-15 inches away and that they need a minimum of 20 min per day. One bunny, Dutchess, had terrible tooth problems until I began giving her daily time outside. The tooth problems disappeared. Recommendations would be really appreciated. Thank you!!

Needing sunlight would be for the vitamin D, which vit D affects calcium levels and so a rabbits bones/teeth. For most rabbits that get daily fortified food pellets with added vitamin D, getting sunlight wouldn't seem as essential, provided the rabbits are getting an adequate amount of vitamin D from the pellets and the pellets aren't too old with degraded vitamin content.

Your rabbits dental issues seeming to go away with time outside, could that be related to being able to eat fresh grass and forage while outdoors, which would provide added tooth wear from this vegetation? Or is your rabbit not eating significant vegetation while outside and you feel the improved dental health is directly related to the exposure to sunlight?

JBun, thank you for your comments. The bunnies do have fortified timothy pellets with Vit D and get lots of timothy hay and veggies inside and outside (kale, carrot tops, a tiny piece of carrot, dandelion greens, green leaf, parsley). The tooth problems only stopped when I started putting them outside everyday, where they get some sunlight. I read in a British vets book that sunlight would decrease tooth problems and she was right. Two are lionheads and one is part lionhead so tooth problems are a concern. Have found a floor lamp on Amazon but am trying to decide on the right uvb bulb. I use Reptisun 5.0 bulbs with my treefrogs, Pac-Man frogs and lizard but they are not powerful enough for the bunnies. Have ordered various entertaining toys and chew toys for them, including musical baby toys that they seem to like. Wish the heatwave would stop so the poor bunnies can start going outside again. I really think sunlight exposure is critical for bunnies’ tooth health.

I don't have any experience to share, as this isn't something I've had to use for my rabbits. But I'm glad you found a solution to your rabbits dental issues. If you aren't able to find a definitive answer to the amount of exposure needed, all you can do is make your best guess.

Hm, I don't think direct sunlight has that much influence on tooth issues. Especially if it's only missing part time, like during heat waves.
Here, farmers kept rabbits in closed sheds and rooms, some still do that, with little light at at all - my grandfathers shed didn't even have a window. Dental issues are a major issue, so I don not think there were this kind of problems, even if those living conditions are squalid health still usually was a major goal, there is no yield from sickly lifestock.

On the other hand, that does not give a hint if sunlight helps with existing tooth issues, since rabbits with issues would have been culled back then.
But nibbling all day sure would help - my rabbits eat differently in the hutch and outside. They gobble down whatever I give them, but outside the grass is short, and they have to do a thousend nibbles with their incisors to get a mouthfull.