This page is a rough attempt to show how geographical location and meteorological conditions affect the amount of solar wattage one needs.

Reminder: available sunlight (insolation) is expressed in Hours of Full Sun Equivalent (FSE) compared to lab conditions. So an entire cloudy day might provide only 2 hours of FSE, meaning it would be the same amount of sunlight as 2 hours of full sunlight under lab conditions. Historical weather info is also used to calculate FSE, so good/bad weather is already baked into the average.

Ambient temperatures also affect panel output, and that is silently factored into the bottom table.

These locations represent the least, average, and most sunlight available in the Continental US. The data is taken from this table.

Worst/lowest FSE tends to occur in December, and most/highest FSE in July.

Note that during periods of poor insolation Phoenix has over 3x the sunlight as Seattle. In summer though, it only has 1.2x as much sunlight:

“The difference in bright sunlight between the least efficient and the most is little. What a panel does in cloudy conditions can make or break your system.” – jimindenver²⁾

These are crude generalities. Assumptions:

• AGM deep cycle battery is discharged to 50% DoD nightly
• MPPT controller is 95% efficient
• PWM yields, overall, 85% of MPPT yield. PWM yields will be quite close to MPPT yields when other forms of charging is present, as PWM will no longer be hamstrung by low Vbatt.
• isolator charging is done in early morning at ~C/3³⁾, and discontinued when alternator no longer raises voltage.⁴⁾ The chart assumes at that point ~50% of charging will be done.
• DC-DC (or generator) charging is is done in early morning at C/3,⁵⁾ and discontinued when Vabs is achieved⁶⁾. We assume at this point 63.3%% of charging will be done.
• temperature-related derating baked in based on monthly average high temperatures
• solar-only charging the same number of amps into lithium would require about 15% less panel than lead. Lithium can be charged successfully off alternator/generator only so no panel would be required.