watts per gallon?????

[tanker newbie]

ok i know that watts per gallon is simply put how many watts of light is being emited into the tank per gallon, but what my question is my sand bed is 3 inches deep, i also have 21 pounds of live rock ... with all that rock in a 20 gallon tank plus the 3 inch sand bed would my wattage per gallon change with less rock or more? more sand or less? or does it phyically go by the amount of gallons the tank is with out anything in it? the reason why im asking is cause if i ever plan to get any corals and i have my retro fit kit in (smart light 55 watt ) if its JUST the tank with nothing in it , then my watts per gallon is 2.75, but if it does include sand and rock then my watts per gallon could be 3.2 or more(based on 17 gallons of water) can anyone help me figure this out, thanks in advance

 

[bobj]

The watts per gallon rule is based on the amount of water in the tank.
However,if you are putting corals in the tank this formula would be better.
Watts needed is equal to the lux required times the area of the tank in square meters divided by the bulb efficacy times the utilization factor.
The efficacy of the bulb is lumens per watt.
The utilization factor is a constant 0.5
This does not apply to metal halide lighting.
10,000 to 12,000 lux is a rough starting point.

 

[tanker newbie]

not trying to disect your post but im a little confused, im a pretty simple guy and i think the answer is a little to in depth . i understand that the intensity of light varys with the depth of the tank but i dont see me haveing any problem with that, my tanks 20 gallons and its the long style,so there fore with a 3 inche sand bed distance to the lights is only 10 and a half inches

 

[bobj]

A FO tank requires enough light for the fish to see and for you to see the fish.
When you add invertebrates,algae,soft and hard corals the lighting requiremants change.
2 to 4 watts per gallon is a rough estimate for the lighting rquirements of a invertebrate non-coral tank, usuing full spectrum bulbs.
Corals require different light intensity. Lux not watts are the measure of light intensity and that's what that formula is about.