how many more watts would it equal?

[Riona]

I've got a 60 gallon tank which has a 120 watt pc fixture over it. The fixture is currently just a retro which I haven't attached to anything, and was an afterthought really since I needed a lot more light over the tank. When I got the tank it had those crappy little 15 watt lights on it and I've got the pc fixture sitting on top of the glass that came from. I was just wondering how much extra light I'll have compared to now actually going into the tank once I make a hood for the lights so it is only over water instead of over glass?