Ok, here is your answer short and sweet and to the point. The standard for reef tanks 4-5 watts per gal. The “K” is a rating of Kelvin. Kelvin is color temperature of the light source is the color of light emitted by a "black body" when heated to a certain temperature measured in Kelvins (K). As the heat of the material increases it undergoes a color change from the warm colors to cooler colors. (see pic below)
10k = yellowish lighting 20k = bluish lights. NOW if you go 10k lights, some type of actinic 03 blue to even out the color spectrum. Depth rate, 150-175 watts for 18 to 30 inch’s deep. 250 watt for 24-36 inch’s deep and for 24-60 inches deep you will need 400-500 watts. Depth rate numbers run into each other for the three level placement, being top for the strongest light middle for medium lighting and bottom for lower light. For every two feet of tank length you will need one 1 metal halide. Now, all this is the “general rule of thumb, type of coral and invert you choose to keep will be the deciding factor. Keep and stick to the general rule and as you become more acquainted and familiar with the reef needs and wants and you do your home work and research the needs and wants of your inhabitants you will run into the “par” term and you will figure it out. So their you have it, a simple easy to follow uncomplicated way to understand lighting for a reef tank or any fish tank for that matter even plants.