Current retail cost for residential electricity if between 7 cents and 16 cents per kWh.
There is no practical way to compare these two numbers. Solar is spotty in its current draw verses an always on power plant.
The .30/watt would be peak production at noon in the desert close to the equator. For real places in the US, the figure will be lower. I'm going to guess that a rooftop unit in California or Arizona would see the equivalent of 4 hours at ideal conditions, so figure .30 gives you 4 watt-hrs/day. So 4 kw-hrs/day would cost $300 and return 40 cents/day (if power costs 10 cents per KWh), or $146/year, for a two year payback
If we assume that the cost of mounting and installing is twice the cost of the panels, then payback would be a reasonable 4 to 5 years
I could envision a "grid" where every "big-box" retailer in the US (Wal-Mart, Target, Home Depot, Lowes, etc.) had a roof covered in solar panels owned by power companies. There are several MAJOR benefits to this:
1) Economy of scale for capital and operating costs.
2) Electricity would be generated where it is needed, reducing the need for additional upgrades to transmission lines. (i.e. Big-box retailers are located near population centers.)
3) The grid would be incredibly terrorisn proof. Thousands of independent generation points.
We are not there yet, but this film might be a step in the right direction.
My electricity here in California typically runs $0.37 a kWh because I go way over the base rate (as do most people here). 1000 hours and it pays for itself here. The real question is how quickly does it degrade. Standard silicon solar panels have a typical life of around 25 years and is degrading all that time. If it takes a year or two to recover the costs that would be a really good deal even if it only lasts 10 years.
If this is really true, sign me up!