normally lights are measured in Kelvins (light "temperature") rather than watts. higher on one light spectrum scale is for groth of plants etc, then lower on the other scale (blue scale etc) for coral growth because they need the light for photosynthesis.
Light intensity is measured in lumens. Power consumption is measured in watts. The color spectrum of a bulb is measured in degrees kelvin.
As to the glass tops question: Yes, they do cut out some of the light, especially in the higher end of the spectrum. As has been said, the dirtier the glass, the more loss you will experience (obviously). There are other materials that are more transparent to light (acrylic, polycarbonate) but they tend to warp too much to use as lids.
If this is for plants, then WPG is useless. Kelvin is for color, not what you want. Lumens are useless too, they're are a measure of the perceived power of light, according to the human eye.
What matters for plants is PAR (photosynthetically active radiation). Just because you have lumens, doesn't mean you have PAR. This is the only real and true measure of lighting as it relates to aquatic plants. Everything else is hogwash. The only way to measure PAR is with a PAR Meter.
But to answer your question, yes, having glass tops will slighty reduce the PAR intensity (measured in micromols) as it travels through the water down to the substrate.
