• Rated = the actual panel rating in watts under lab conditions
• MPPT theoretical = 82% of rated, allowing for 5% DC-DC losses and 13% temperature derating at 70deg F ambient.

In my case, Rated is 570w and MPPT theoretical is 467w.

Overpaneled configurations (relatively high panel watt to battery Ah ratio) tend have two main features:

1. improved charging in adverse conditions
2. apparent low output in normal conditions - this occurs because the batteries are typically in Absorption with battery charging demand dropping by solar noon; there is rarely an opportunity to demonstrate full output.

• full sun normal
  • 305w, 54% of rated
• bright sun through thin / cirrus clouds, shadows thrown
  • 312w at 11am, sea level. 55% of rated, 67% of theoretical.
• bright overcast, sun is completely occluded but too bright to look at comfortably, no open sky, no shadows - 25% of rated, 67% of theoretical
  • 150w spiking to 200w in light rain (26-31% of rated, 33-44% of theoretical mppt)
• overcast, can look at sun's assumed position - 20% of rated
  • 125w, 22% of rated
  • 100w, 18% of rated
  • 75w, 13% of rated
• dark overcast, cannot locate sun - 10% of rated
• exceptionally low yield - less than 5% of rated
  • thunderstorms, typically 10W, less than 2% of rated.

This is harder to grasp and compute, but increases accuracy by factoring in location, time, etc.¹⁾

1. find solar zenith angle with app or calculator
2. derive cosine of the above; this is the percentage of direct sunlight landing on a flat surface
3. multiply this value by panel wattage
4. the result is the theoretical direct light available at that time/place under clear skies.²⁾

I have observed the following cosine-based direct light percentages:

• clear skies: 98% of theoretical direct sun
• bright overcast skies (shadows thrown): 82% of theoretical direct sun
• overcast: 58% of theoretical direct sun

• exceptionally high yield, 90% of rated
  • 519w at in February in Quartzsite @ noon, ~500' elevation. 91% of rated, 114% of theoretical. I've seen 500w+ out of 570w rarely, maybe 3x in two years off grid so far.³⁾
• wildfire smoke - 8.98W at noon, sea level. 1.5% of rated, 1.9% of theoretical during September 2020 wildfires in Oregon.