The insolation requred to get decent return is often overstated on the internet - you generally see over 1000kwh for a 1kw system over a year, only dropping to ~900 in scotland. California only really gets 40-50% more useful energy out. Sure, it's less, but often close enough that it can still make sense - and more dependant on local energy and PV system costs than anything else.

The idea that you need constant, overhead, wall-to-wall sunshine isn't really true for current systems. In fact, somewhere nearer the equator the effeciency drops significantly for PV systems (even if the total power output increases slightly) as the "extra" light isn't really useful.