Except none of that matters, Solar Irradiance has very little to do with actual photosynthesis. You are measuring Watt's as a pure energy source, that's useless in this case. Photosynthetic photon flux density (PPFD's) are what's important, that's the amount of light wavelengths that the plant needs to absorb to power photosynthesis. Here is a definition from another source, I'll post the link at the bottom:

PPFD is a measure of photosynthetic active radiation, PAR for short. PAR is not a measure of anything itself but is more of a description. PAR light is all the visible wavelengths of light which cause photosynthesis, found within the 400-700 nanometer range. PPFD is a ‘spot’ measurement that tells you how many photons from the PAR range hit a specific area of your canopy over time. It is expressed as micro moles per square meter per second (μmol/m2/s). For this reason, PPFD is the most accurate measure of light power. First, unlike other measures, it considers the entire spectrum of light that plants see. PPFD also takes into account the amount of light that will actually reach the plant instead of focusing only on the point of origin. A light source can be very bright and powerful, but if it is too away from the plant, or obstructed in some way, the plant won’t be getting all the light it needs for photosynthesis. PPFD controls for this kind of inaccuracy.

With LED's you can place the plants much closer to the light source and control the color spectrum along with the obstructions 24/7. It allows you to grow indoors at an extremely efficient rate. Your formula has almost nothing to do with indoor farming.

https://www.freightfarms.com/blog/indoor-grow-lights#:~:text....

Here are some additional sources: https://archipel.uqam.ca/1619/1/1998_025_Gendron.pdf https://www.ncbi.nlm.nih.gov/pmc/articles/PMC1066711/pdf/pln... https://www.researchgate.net/profile/Paul_Hanson7/publicatio... https://www.horti-growlight.com/en-gb/par-ppf-ypf-ppfd-dli#:.... https://www.sciencedirect.com/topics/medicine-and-dentistry/...

Quick summary: PPFD measures the amount of light wavelengths that the plant captures that kicks off the chemical reaction of Chlorophyll. Red and Blue lights are the vast majority of wavelengths that interact with Chlorophyll. That's a major reason why the solar irradiance measure of pure Wattage is irrelevant.