Solar harvest under 100% overcast / tilted v. flat

Today is 100% overcast with medium light. There are no shadows and you can’t tell where the sun is in the sky by casual observation. It’s not optimal, but this is why we overpanel.

We would expect a couple of things under such skies: reduced harvest compared to clear skies, decreased delta between tilted and flat-mounted panels as the path of sunlight is diffused, and relatively insignificiant additional harvest from the 35% increase in wattage from the portables since 35% more of a small number is still a small number.


  • mounted panels - 155.14w (27% of rated)
  • portable panels at daily optimal tilt - 53.6w (26.8% of rated)

Close enough to be a wash. Yes, +54w is nice. No, it’s not life-changing.

I’d expect to see about 30% of rated in the next hour as we reach solar noon, assuming the medium overcast holds and we don’t reach Vabs by then. {Update: reached Vabs 14.7v at 12:02p, a half-hour before solar noon}.

A smidgen of bright overcast

Every once in a while light levels increase to what I call “bright overcast”, where you can make out the location of the sun in the sky. With more direct sunlight the tilted panels should start to see an advantage.


  • mounted panels - 169.68w (29.7% of rated)
  • portable panels at daily optimal tilt - 65.6w (32.2% of rated)

Here we see the tilted panels starting to make +10% over their flat-mounted brethren.