Even at $1.60 per watt material cost, given power rates in my area, daylight hours, and angle of the sun, it would take a little over ten years to produce $1.60 worth of electricity (22,857 hours at an average of 6 hours per day of peak output). There is no pay back for solar (unless you count grants and tax write offs, but the product cannot pay for itself.)
Nothing wrong with your rough numbers, however, you fail to understand the real plan at work here. Those that intend to force you to use their particular brand of Politically Correct power in the future don't have to drive down the cost of solar. Their plan is to simply and drastically increase the cost of power that they disapprove of. No matter that coal, nuclear, and natural gas could provide the USA with with cheap, plentiful domestically sourced power for the rest of our lives, we're going to have to buy overly expensive and unreliable solar because our political masters are beholden to wackos.
Wackos, I should add, who will profit immensely due to their investments in uneconomic (until you're force to buy it at their prices) Green Energy ventures and carbon offset farms. You're about to be taken for the most expensive ride of your life here -- and in Britain as well.