It’s just one analyst, but that will be an interesting tipping point:

“Solar PV modules are increasingly becoming commoditized as costs steadily decline and research paves the way for further cost reductions. Module costs have fallen from around $4 per watt in 2006 to below $1 per watt in some regions by 2012, driving solar power toward grid parity in mature retail energy markets.

‘By the end of 2020, solar PV is expected to be cost-competitive with retail electricity prices, without subsidies, in a significant portion of the world,’ said Dexter Gauntlett of Navigant Research.”

(Via Solar PV To Hit Grid Parity, $134 Billion Annual Revenue By 2020.)