predicting the sun’s path

Yesterday I badly misjudged when the sun would “set” behind the mesa to the south. Using the manual method I judged the sun would start to be hidden around 3:30pm; in acuality I had direct sunlight until ~5:15p. Now there’s not a lot of power available when the sun is very low in the sky but we take what we can get.

So how was I so wrong? Two things:

  1. I forgot how (apparently) lateral the movement of the sun is in the winter.
  2. I didn’t notice that lateral path would intersect the terrain at a much lower point nearby, effectively granting more sun.

After enjoying that rare lucky break I remembered there was an app on my phone that knew all along. Sun Surveyor is an augmented reality app for Android that overlays a camera view with data about the sun’s path. Like so:


I’d been basing my estimation on the height of the terrain on the left side of the pic, assuming the sun would go straight down. Duhhh. As you can see, it continued to sweep across the sky and finally descended behind much lower terrain.

I’d used the app before on my old phone but totally forgot about it on the new one. In summer in open spaces it’’s not that useful. But in winter or near obstructions it can be critical for planning panel/van placement. I don’t remember the difference between the “lite” and full versions, but I had credits from Google Rewards so got it for “free”.

bonus metrics from this morning

  • 0830 - solar “break even”, enough light to exactly balance loads. Not charging, but not discharging either. 0w in/out.
  • 0851 - the 12v opportunity load circuit kicked in, so we know Vbatt was >= 12.9v for at least 10 minutes (that’s how I presently have it defined). The panels really couldn’t support the entire opportunity load yet – Vbatt slumped below 12.8v after a couple minutes and the circuit deactivated. This suggests a longer delay might be warranted in winter if I needed the opportunity circuit to be consistent (I don’t).
  • 0909 - opportunity circuit came back on and stayed on; we now have enough power to run opportunity loads and charge the bank. This is 18mins later than the first activation, suggesting a delay of ~30mins might be optimal for consistent operation. Increased delay doesn’t matter to MPPT, but folks with PWM controllers will see higher output when Vbatt is allowed to rise higher.
  • 1023 - as I finish writing this post the sun is power the opportunity loads and putting 184w into the bank. Total harvest is 322.42w, about 42% of total panel wattage.

    861 days boondocking 823 days on 6v GC2 bank