[walrus01]

That is a really good answer, and thanks for taking the time to write it up. The difference in instantaneous wattage produced by good quality 156mm monocrystalline Si cells is huge in direct sunlight vs shade. You can take a modern 60-cell, 300W rated panel (STC rating) and hold a piece of cardboard a meter away from it, to temporarily shade it, and see a huge difference. What size of PV panel are you using for each node, and how many/what type of cells?

I'm assuming that for cost reasons and economies of scale that you're using some sort of standard off the shelf PV module that can be acquired in bulk, at low cost, such as a 156mm poly x 54-cell module. Or maybe something a bit smaller than that.

1.5W load x 24 hours x 31 days = 1116 Wh per month

For a typical node what is your calculated PV production in Wh per month? Taking the 40Wh per day figure mentioned above, that's 40 * 31 = 1240Wh per month production goal from the PV. Will of course vary with mounting location and shading, but I'm sure you have a floor figure that you don't want to go below, or batteries will reach a LVD threshold overnight and everything will shut off.

I'm also making an assumption that per-node cost is somewhat of a concern in general. For very small PV setups a decent charge controller (that can handle up to the max of one medium sized panel) is not very expensive. Are you using an off-the-shelf $65-75 chinese small MPPT PV charge controller with voltage levels set up to match your LiFePo4 battery setup, or something you've designed yourselves? something like this? https://www.amazon.com/DROK-Controller-Converter-Regulator-C...