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Words of Wisdom: “I'm not sure people always grasp the concept of “minimum” either; they tend to think it means “all that you really need” when it fact it means “the least you can get away with provided everything else goes right”. After all, the sun does not shine brightly every day.” – Cariboocoot1)

Solar panel sizing for different geographical areas

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.

representative areas

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.

Location Average FSE Worst FSE Best FSE
Seattle, WA 3.53 0.90 6.27
Salt Lake City, UT 4.53 1.97 6.98
Phoenix, AZ 5.38 2.75 7.7

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)

Amount of solar wattage required per 100Ah of Pb

These figures have temperature-related derating baked in, then are rounded off to usable numbers.


  • 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, and discontinued when alternator no longer raises voltage. At this point ~70% of charging will be done.
  • DC-DC (or generator) charging is is done in early morning, and discontinued when Vabs is achieved. At this point 85% of charging will be done.
  • wattage listed as any means “any amount over 100w”.
  • 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.
Location MPPT only PWM only w/ isolator w/ DC-DC charger or generator
Seattle - avg 250w 300w any any
Seattle - Dec 950w 1100w 275w 150w
Seattle - Jul 150w 175w any any
SLC - avg 200w 250w any any
SLC - Dec 425w 500w 125w any
SLC - Jul 150w 175w any any
Phoenix - avg 175w 200w any any
Phoenix - Dec 325w 400w any any
Phoenix - Jul 125w 150w any any
opinion/frater_secessus/panelsizesforinsolation.txt · Last modified: 2020/10/11 19:48 (external edit)