Distributed Solar System
There are two main sources of solar energy: centralized plants, called Concentrated Solar Power, and distributed rooftop systems. Here we focus on distributed rooftop systems which can be put on top of buildings. Obviously these systems only work when the sun is out; each location has different seasonal sunlight, and systems work best if they can move to follow the sun. We will use rough average estimates here. Also, the price of photovoltaic (PV) systems continues to decline, but we will be conservative in our estimates. Note that if buildings also had batteries, they could store the solar energy for use at night; this would be particularly useful if the building also incorporated a geothermal heat pump, as the solar panels could power the heat pump at night via the batteries, so that buildings would use very little electricity from the national grid. This would not only free up the grid electricity to be used by industry and transportation, but it would also decrease the possibility of going without power in the case of a blackout.
If we are very conservative and assume the weather of cloudy Seattle, about the worst in the continental U.S., at about 26 cents per kilowatt/hour, using the National Renewable Energy Laboratory (NREL) figures from 2010 (page 52), and if we wanted to supply about a quarter of the country's current electricity, or about 1,000 TWh (terra watts, or trillion watts), then we would need to spend about 3 trillion dollars over 20 years (assuming that it would modules would last 20 years, thus we would start over at the end of 20 years to replace old systems). That would equal to $150 billion per year. Note, however, that if the average for the U.S. was closer to the 20 cents per kilowatt hour of Kansas, then the total cost would be closer to $2 trillion over 20 years, or $100 billion per year. Also note that prices for the actual solar panels themselves have been decreasing and will continue to decrease.
According to the Solar Energy Information Industries Association, in 2015 about 7 Gigawatts of solar capacity was added to the U.S. There were about 209,000 solar industries employees. Say we add 40 Gigawatts capacity per year, which would equal 800 gigawatts at the end of 20 years, which would yield about 1600 terrawatt hours, then we need about 1.25 million workers, with about one quarter for manufacturing, or about 300,000.