Are UVA leds effective to kill fungus or is sunlight better (it contains UVA and UVB)?
What is the recommended power?
I don't think that there are LEDs that are in the germicidal UV range. All the germicidal UV lamps I have seen have been mercury vapor, which is basically a flourescent without the phosphor coating on the inside. They also have to be made from quartz, not glass, since ordinary glass will absorve these wavelengths.
I don't know the UV properties of glass used to make lenses, but most likely it won't transmit the short wave UV anyway, whether you make the UV rays with a mercury vapor tube, LED, or some other method.
Apparently some LEDs are available in the wavelength range that will probably kill fungus, as noted by Caleb in a comment below. However, the problem of getting that UV thru the lens glass to the fungus is still a problem.
Perhaps prolonged exposure to longer wavelengths that the glass can transmit will kill the fungus too, but I don't know that. I suppose there is little harm in leaving the the lens pointed at the sun for a prolonged time, as long as you take care to make sure the rays don't end up focused anywhere they could cause trouble. You'd probably have to use something like a telescope mount to keep the lens pointed at the sun for a few hours.
In any case, killing the fungus already inside the lens at best just won't make it grow bigger. Whatever fungus is already there, with its associated optical degradation, will remain.
Silica Gel and UV/Sunlight
This is not an answer, but un-answer rather. The two vastly popular "solutions" for fungus are silica gel (as preventive measure) and uv or sunlight to actually kill the fungus. So I will provide the common answers, and explain why they're is wrong.
Silica Gel as a preventive measure - it works by absorbing the moisture/humidity which the fungus requires to grow. Now you have to understand that silica gel is not a mechanical device that can just keep going, it's just a chemical. Once it absorbs the moisture to its maximum capacity it completely stops being useful. For shops and people who wish to store their lenses it is of course a great solution. But for active photographers (who will frequently open and close their camera bag in humid environments) it is completely useless. You will get this advice in forums and photography stores. But it's just wrong. There's no even a "could" protected. Just no.
UV/Sunlight as a way to kill the fungus - While I haven't personally tested what happens to the fungus when you shine the light on it or leave the lens for some period of time under direct sunlight. I am fairly certain (to about 100%) that it will not fall off. It will just stay there. So, you must open the lens for two reasons.
1st, because you can't be sure that the duration of light actually killed the fungus. I mean, you're not going to be checking up on it from time to time to see if it's continuing to eat up at your lens and growing slowly right? You are dealing with a microscopic entity, shining a light on a sealed lens will most likely have places inside which are hidden from the light, and spores can survive there.
2nd reason you will want to open the lens is because unless the fungus is super tiny (and once you've noticed it, it's usually not) it will affect the image quality to some degree and need to be cleaned off. So, in other words, this is not a solution that will avoid disassembling the lens. It's not a solution at all, in fact. Because if you will be opening the lens anyway, why bother with this voodoo stuff? You will have full access to the internals and have more direct ways to combat the fungus.
I should mention, that direct sunlight has a very high chance of damaging the electronics inside the lens. Also, as the sun moves, an angle could shift causing a concentrated beam which could possibly damage something as well.
So please, dear photography enthusiasts and professionals. I'm sure you're aware of the extent these two "answers" have spread throughout the virtual and real world. Vast majority of the people providing these two solutions have not personally tested or given much thought to them. Lets try to "roll it back" a little bit, and discourage these two answers as much as we can.
The only reasonable solution would involve direct internal application of some sort of chemical. On that too there are various vague suggestions (but I won't get into that). Fungus is relatively easy to remove with just light soap and a cloth. I've done it, and it was gone for a year. I even added a very light ammonia to the mix (too light perhaps?) However it returned the next year (now), coinciding with the cycles of tropical environment of the country where I'm staying. Initially all my 3 lenses and even the camera sensor were affected. The sensor was completely covered when I noticed the problem with the lenses. Although the sensor is still clean now, all 3 lenses got the fungus like a clock at the same time. A real solution would involve something that keeps it away. Or at least kills it permanently. So if you do get it again it comes from a different source. Clearly in my case I didn't kill it thoroughly.