Just did some real world research. https://www.gogreensolar.com/products/30000-watt-30kw-solar-panel-ground-mount-installation-kit?s=recomatic
30 kW system takes about 2000 sq feet and would cost just south of $100K installed with no batteries. It throws your numbers into question.
15 watts per square foot at 20% efficiency would mean 75 watts solar energy per sq foot reaching the ground. About 800 watts per sq meter.
There are Watts, and then there are Watt-hours. Watts are power, and Watt-hours are energy. If the Wikipedia numbers are for a 24 hour day, 150 continuous watts per square meter, that would give you 150*24 = 3,600 Watt-hours ( 3.6 kW-h) per day, per square meter of sunshine energy, if the panels track the sun. You have to also factor the solar cell efficiency, which is roughly your 20% number for commercially realistic cells over their lifetime. The lack of tracking mechanisms will significantly reduce that.