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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 (1000w/sq meter). Therefore 2 hours of FSE = 2kWh.2) Multiply [FSE for the time/place] x [rated panel wattage] to see what see how many watts your system would harvest on average if it were perfectly efficient.3). Example: 4 hours of FSE x 200w = 800Wh, or 0.8kWh, before various losses.4)

Historical weather info is also used to calculate FSE, so good/bad weather is already baked into the average.

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.” – jimindenver5)

Amount of solar wattage required per 100Ah of Pb

These are crude estimates about the panel wattage required to fully charge 100Ah of AGM from 50% DoD each day. See the list of assumptions below the chart.

Location MPPT only PWM only w/ "isolator" w/ DC-DC charger or generator
Seattle - avg 280w 329w 140w 103w
Seattle - Dec 921w 1056w 528w 388w
Seattle - Jul 163w 192w 82w 60w
SLC - avg 238w 281w 119w 88w
SLC - Dec 473w 556w 236w 174w
SLC - Jul 156w 183w 78w 57w
Phoenix - avg 200w 235w 100w 73w
Phoenix - Dec 369w 434w 184w 135w
Phoenix - Jul 149w 175w 74w 55w


  • panel output is temperature-derated according to daily average high at each time/place
  • no other loads other than charging are present
  • AGM deep cycle battery is discharged to 50% DoD nightly and fully charged the next day. Charging efficiency 90%.
  • 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, every morning, at ~C/36), and discontinued when alternator no longer raises voltage.7) The chart assumes at that point ~50% of charging will be done.
  • DC-DC (or generator) charging is is done in early morning every morning, at C/3,8) and discontinued when Vabs is achieved9). We assume at this point 63.3%% of charging will be done. The practical difference between alternator charging and DC-DC or generator charging is the latter will charge all the way to Absorption voltage.
  • temperature-related derating baked in based on monthly average high temperatures
  • due to lower internal resistance lithium will accept more charge from alternator/generator than AGM by the time it reaches the charging source's voltage setpoint, thereby requiring somewhat less panel wattage. Likewise, flooded batteries will accept less than AGM and then require more panel.
  • solar-only charging the same number of amps into lithium would require about 10% less panel than AGM due to better charging efficiency. Lithium can be charged successfully off alternator/generator only so no panel would be required.

further reading

and 7.2MJ (megajoules) for our metric friends
hint: it's not
see Assumptions section at bottom for typical losses
6) , 8)
~33A, AGM max charging current
opinion/frater_secessus/panelsizesforinsolation.txt · Last modified: 2024/03/20 00:01 by frater_secessus