This amount is adequate however I wouldn't trust them enough to believe that the bulb is actually producing this amount of UVB, my best advice to you is to ask Frances from uv guide about this bulb or you can just order a reptisun 10.0 which is considered as a good uv bulb.
i bought one of these a while back and no, its garbage, at least the bulb is. buy a reptisun 10.0 and put it in it.
i'm not sure how much it actually puts out as i don't have a meter but judging simply one the vast differience of light output between the zilla bulb and a reptisun i bought i would say its not very good.
the older version of this fixture was also notorious for causing eye problems in reptiles, they supposedly changed it but why risk it
here is the site that has test info on the old version of the lamp
50w/cm2 was read by them at approx 5" distance when the lamp was new with the plastic shield and about the same range with the shield removed after 105 hours of use (break in period) so that proves that their previous version was not very good, compare that to the reptisun 10.0 24" they tested it has well over 80w/cm2 at the same distance with no reflector (the zilla has a reflector in it that increases its UVB output by close to double according to tests they have done on that site with reflectors.
yeah, its the bulb that is potentially a problem, a fixture is a fixture for the most part, just make sure to get a 18" T8 reptisun 10.0 (not a reptiGLO)
the old version as well as the old versions of both reptisun and reptiglo compact florescent bulbs (not tube ones) outputted too much UVC light that is unnatural in sunlight and damages cells, the eyes of reptiles are the most sensitive naturally so the ones that were causing this problem would cause various reptiles to close their eyes when under these lights and in severe cases cause eye damage
all of the companies have claimed to have fixed the problem but the fact of the matter is that the tube florescent's work the best and are the safest to use esp reptisun's that is why everyone on here uses reptisun 10.0's because they have always worked and are tried and true
plus if your going to spend 30 bux on a bulb which bulb would you rather buy a cheap exo-terra bulb that may or may not output enough UVB and is made in china or a zoomed reptisun 10.0 that you know will output enough UVB at the right distance and is made in Germany