There's a good treatment of lights in the greenhouse forum. Lumens are a misleading measurement because what's actually measured is light percieved by the typical human eye. Our eyes pick up a different spectrum of light than that which is used by plants for photosynthesis, so a bulb could have a very high lumen rating and still provide little useful light to plants. If Tony says you're good, I'd believe it. I grew my first Nep on a single 18 watt bulb and supplemental sunlight for a little more than a year. It didn't kill the plant, but it wasn't growing very fast either. When I switched to forty watt bulbs, I got much more satisfying results. Faster growth, better coloration, etc. But I also went from one small bulb to four big bulbs, so that probably played a big role as well.
Good luck,
~Joe