In what areas of the United States would solar energy be most effective?

If you only look at solar radiation, the states in the southwest are the most promising. However, once you factor in cash rebates, expensive electricity and financial incentives… states like New York, New Jersey and Massachusetts are seeing plenty of solar PV installations without being particularly sunny.

In places with expensive electricity, such as New York and California, even solar power + storage is now financially viable. It makes sense to store surplus energy from solar panels during daytime to avoid the hefty electricity rates charged during peak hours after sunset. In fact, this has a double benefit: utilities can avoid a phenomenon called the “duck curve”, where they have to handle surplus solar generation around noon and a demand peak after sunset.

Source: http://www.quora.com/In-what-areas-of-the-United-States-would-solar-energy-be-most-effective