J Fortuna wrote:
We had been seeking fixtures rated for 40-watt bulbs, based on previous
posts here. But we found all the 48" fixtures rated for 40-watt bulbs, the
36" fixtures for 30-watt bulbs, and the 24" fixtures for 20 watts. Are these
ratings really based on length? Is 30 watts over 36" sufficient intensity?

Note that each of your example tubes consume 10 watts per 12 inches.
Each outputs approximately the same amount of light per linear inch. So
while a 40 watt tube outputs approximately twice the lumens of a 20 watt
tube, it does so over twice the length, so the light per unit length is
identical.

Put another way, 40 watt tubes will illuminate 4 feet of orchid bench,
30 watt tubes will illuminate 3 feet of orchid bench, and 20 watt tubes
will illuminate 2 feet of orchid bench -- with the orchids in each case
illuminated with approximately the same intensity.

John