I have little doubt that they can do without power longer than you can. Nothing to worry about if you ask me. Just the equivalent of a spate of cold weather in nature and we all know how chilly that can get.
UVB has nothing to do with protein. UVB light allows dragons to photosynthesis Pre-vitamin D in their skin (from a form of cholesterol). This is then stored in the liver until needed, where it is converted into active Vitamin D. Vitamin D is required by the body to allow it to metabolise (i.e. use) calcium. It is needed for the uptake of calcium from the gut following digestion. It is needed for the incorporation of calcium into bones to strengthen them. It is needed for the utilisation of calcium in the transmission of nerve impulses. It is needed for the role of calcium in muscle contraction. The list goes on.... Calcium is a critical mineral in a whole range of metabolic processes. The level of calcium needs to be kept relatively constant in the blood for normal functioning of the body. This is why animals suffering from a lack of calcium take it out of storage from their bones.
Insufficient calcium in the diet or a lack of exposure to sufficient UVB and therefore insufficient vitamin D, can ultimately result in the condition known as Metabolic Bone Disorder (MBD). There are other causes, such as faulty kidneys that excrete too much calcium.
Iguanas are similar to dragons in that they derive most of their vitamin D through exposure to sunlight. Some initial experiments have indicated that dietary supplements of vitamin D can be successfully substituted for exposure to UVB, given adequate dietary intake of calcium. The experiment involved only Green Iguanas and several died for unknown reasons during the trials. So given that adult Bearded Dragons have fully formed bones, it may well be possible to substitute their UVB exposure for dietary supplements of vitamin D. Unfortunately, I am not aware of anyone who has run a controlled experiment to that effect. If anyone else is, I would dearly like to hear about it.
Blue