A solar panel in Phoenix is worth more than one in Seattle.
That's not entirely accurate. On a typical hot summer day, it's not uncommon for a solar cell to reach a temperature of 70 °C (158 °F). A general rule of thumb is that the efficiency of a solar cell decreases with 0.5% for every 1 °C (1.8 °F) above 25 °C (77 °F). This means that on a hot summer day, the efficiency of a solar cell could drop as much as 25%.
Sure, you're going to see a lot more sunlight in Phoenix, but it's also a lot hotter there. Even the diffuse sunlight from a cloudy day can produce a fair amount of energy. The amount of energy produced will obviously vary based on setup, but just because one part of the country sees more sunlight doesn't necessarily mean that solar power is more efficient there.