1. How much more on your energy bill for a 400 watt Sodium light?

2. If I were to create a 4x4 box in a room, would I really want to line it with tin foil on the inside? Is that 400 watt light enough for 6-8 plants?

I worry about the neighbors seeing these lights on, thats why I wouldnt want to make the whole room a grow room.

Any suggestions to get me going.

a 400w light on 18 hrs a day will cost you about £2 a week.
how high are you planning on building your box, my guess is that it will have to be at least 6ft tall. to allow for growth of plants and the distance from the top of your plant to the light.
i cut a piece of plyboard to exactly fit into my window frame, i closed the curtains and squeeze the board in and no light escapes. it just looks like your cutains are drawn all the time.

So you think I would want to make the room 4x4x6. So the room should be set pretty high?

keep it high enough for the pots or hydro setup, plants distance between plant without burning them, distance to grow and space for the light. remember to vent the heat and light sould not be a problem cause it sould be air tight (except for in out take) and light proof you dont want light coming in during the dark period. a 400 or 600 watt system will work well try to get a air cooled hood it will help with heat and safty. good luck keep us posted

YES LINE WITH FOIL OR WHITE PLASTIC.REMEMBER IF USING FOIL USE THE DULL SIDE FACING PLANTS NOT THE SHINNY SIDE HOPE THIS HELPS MR.H