Is one 18" 20w T8 bulb enough to start my seedlings?

I am making a seed starter box [like this one: https://www.rollitup.org/view.php?pg=faq&cmd=article&id=431 ] and i had the box lying around. I also have an 18" Under Cabinet light. Will this be enough light to start ~15 seeds in an 24"x4"x8" box [lxwxh]?

Sure, for the first week or so but after that your going to want more.