by low wattage what do you mean? the wattage really depends on what type of light your using. I would look for at least 100 watt, but it will change with the amount of plants in the room. Also it looks like your growing more than one plant in the same space, I would not recommend doing this. On the other issue Im starting my grow using Led lighting. But if your looking for cheap lighting (not electric wise, but product wise) use some grow lighting or tube lighting.
Ok first i use 100w full spectrum cfl i recommend you try those there like $8.00 at home depot! Second you got way to many seedlings in one cup you should plant one per cup thay need alot of space to grow roots and with that many there going to choke each other out if you have any other questions hit me up oh and check out my grow https://www.rollitup.org/cfl-fluorescent-lighting/595885-c-f-l-grow-la.html