Short answer is that more of the solar insolation that hits Earth gets converted into electricity. So on the equator its about 1kW per square meter, in North America closer to 700W sq Meter, and converted at 50% you would start at 350W per sq meter, which after rectification might be 300W per square meter.

Where nowadays you might put a 2kW installation on your roof (using 14%-ish efficient cells as are currently available), you'd be able to fit a 7kW installation in the same space, start putting 2kW installations on houses with much smaller available roof area, or start using solar effectively in much more overcast areas.

2kW is a usable approximation for the total power usage of an "average" house.

Probably just a benchmark. Goal # 1: "At least get half the way there".