Reply #5: Cost per watt is a rather misleading figure.

[TheMadMonk]

Generally panel efficiencies are quoted for the best case, ie under midday summer sun. (or roughly 1000W/m2 multiplied by the efficiency of the solar cell). To figure out your real costs, you need to know your complete annual insolation profile and then decide what level of independence from the grid you wish or can afford.

Obviously the most expensive option is total independence and isolation from the grid. Something one would only choose if connecting to the grid was impossible or prohibitively expensive.

With the option of selling power back to the grid, the most realistic would be aiming for zero balance or better over the course of the year, using credits earned in good times to cover for the lean. With of course any number of variants ranging from energy mogul to salving your guilt by covering for the air conditioning in summer and the lights in winter.



Oh, and something I just thought of, for conditions that have a lot of scudding cloud, sliver cells http://www.originenergy.com.au/environment/files/factsheet_sliver.pdf are a good bet, since they can deliver power even when partially shaded, in addition to their other advantages. (The PDF says they should already be available, but I believe that there have been problems with automating production and ramping it up to economical levels.)