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.
Just did a spreadsheet and did some back of the envelope calcs. 1500 sq feet of panels with 15% efficiency to H with 5 sunlight hrs a day and 65% end use efficiency would meet most homes energy needs. Assuming double the cost of present solar that is a 7 year break even not factoring in interest cost.