One thing I've wondered, not about making the batteries, but the feasibility of solar cells [[and batteries) in other parts of the country [[e.g., the Midwest, Mid-Atlantic, etc.) besides places like CA, AZ, FL, etc.?

Is the key factor the percentage of sunny or mostly sunny [[i.e., not cloudy) days? Does sunny and 50 degrees work well or does it need to be more 'sun beltish' - bright sun?