While the Southwest region of the United States has some of the best solar resources in the world, almost all areas of the continental United States and Southern Canada have solar resources suitable for solar energy systems. For example, while frequently overcast and rainy Seattle is considered the worst place for solar in the United States (outside of Alaska), it still has a better solar resource potential than Germany which alone accounts for a third of the world's solar energy installations.
In addition, despite receiving less direct sunlight, northern states have the advantage of photovoltaic (PV) systems working more efficiently during cooler temperatures because heat lowers the efficiency at which the panels convert light to electricity. Northern states also have longer days in summer which can contribute to relatively more generation, which depending on when the power is needed, may offset shorter winter days.
Solar hot water systems can also work in more northern climates, typically requiring at least 5-6 hours of sunlight per day for two-thirds of the year, and many systems are designed specifically for cloudier or high-altitude environments.
Economics and associated public policies are arguably more important in determining regional solar uptake rates than solar resource potential. Areas like New England have some of the highest electricity rates in the country meaning that solar might more sense there than in areas with lower electricity rates. In fact, Massachusetts, New Jersey, and New York are all rated among the top ten solar states.