Imagine if I were to place two 48 inch shoplights, each with two 40watt bulbs over a 20 gallon, so that would be 4 40 watt bulbs for a total of 160 watts.

Now the tank is only 24 inches wide, and the shoplights are 48. So only half of the shoplights are directly over the tank. So if only half of the lights are over the tank, I am guessing only 80 of the 160 watts is actually useful.

If it was a 29 g 36 inches wide, would it only 3/4 of the light be useful?