If your area is 1.2' x 1.2' then you have 1.44 sq foot of area, not 15.5 sq ft.
I like to think in terms of lumens, lux or PAR (for LED lighting) That Watts is a real "nothing" when it comes to light. Watt is a measure of energy or power. 1000 watt bulb will burn 1 Kw or electric energy/ hour. Plant's don't care about watts, they care about light output and spectrum. You can overcome the first by distance to plants.. closer is more intense light. You can only overcome light spectrum by the light you buy. It will only and always have what it was designed for in spectrum. My one 40W SANSI light puts out 35,800 lumens at 1 foot from the light. The lumen goes goes down pretty quickly the further you get away from the bulb, but that is the case with all artificial light. EXCEPT if you grown in a confined area that has full silver reflective material on the sides, bottom and top. More lumens are available for the plant further down from the bulb (source) in this instance.
A surface area of one square meter on Earth receives typically about one kilowatt of
sunlight from the sun (the
solar irradiance) (on a clear day at mid day, close to the equator) source :
https://en.wikipedia.org/wiki/Watt Just an example...
The plants I grew under those two lights thrived and grew strong and green. Finally got a chance to put them outside when the danger of frost had lessened. They are doing great. I would like a tent someday as a lot of my light is spread out too much and does not have the light the plant needs at it's lower branches.
SANSI light spectrum:
View attachment 1064372
I choose the bulb, not because of the wattage, but because of the spectrum of daylight light it puts out.