The key to using Powersun is proper set up, which itself is not cheap.
View Powersun as a sophisticated and delicate electronic device. The directions must be followed very closely. Most people like to use spot lights at an angle. For those that do this with Powersun, you will find your Powersun going out for good in a short amount of time. It must be pointed straight down. I use not only a ceramic socket, but also a large ventilated aluminum hood outside of the enclosures (many monitor/ lizard keepers set them up inside of enclosures). With this set up, I have not lost one bulb or had temporary shut down. These are used in my reptile room, which is very well ventilated to the outside (for the last 6 months 90 highs with moderate high humidity- always high compared to the US). If there are temporary shut downs, they are caused by improper conditions or excessive heat build up.
The mercury vapor bulbs are far superior in UVB/UVA output to the fluorescent fixtures and there is a significant difference in some of my reptiles that have the Powersun which includes improved color, appetite and increased activity. One warning: there are some brands mercury vapor lights that may have too high of an output and have actually caused eye damage in some specimens; I am not sure whether this was caused by 24 hour lighting in all cases or not.
There should be no danger of vitamin D3 overdose from using artificial lighting. The only cases of internal calcification due to vitamin D3 overdoses that I have come across in research have been from excessive vitamin D3 supplements together with natural sunlight. I have never heard of a case where supplements given as directed have caused this problem even with the combination of natural sunlight and vitamin D3. If anyone knows of this happening in a case other than this, please let me know.
I personally do not use Powersun for my tortoises, but then again, I have the luxury of the ability to house my tortoises/ place my hatchling tortoises outside all year round. I use them for my young monitor lizards and agamids.
Technology is constantly improving in reptile lighting, but nothing compares to natural sunlight! For those in the temperate climates, I know natural sunlight is not an option during the colder months.
Hope this is of help to disgruntled Powersun users or those interested in its use.
Michael