Depends on what the ballast is (is the 18 W marked on the fixture, or the ballast itself?) .... An electronic load sensing ballast will put out enough watts up to the max rating, and prob can go over a bit as well. If the bulb is lighting up, it should be getting closer to 36 W than 18 ... it might be 30 of so. <I don't know the limits for the T5 PC
's, but the T8/12's will not light if the ballast is more than 10-20% underpowered.>
I don't know what lowering the wattage supplied to the bulb will do to the spectrum. I suspect that it will be different, and prob. shifted to the red end.