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)
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.
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|
|Salt Lake City, UT||4.53||1.97||6.98|
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)
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|