Ok, I know we have all looked at our solar output and wondered where all those watts are. Maybe those of us in PNW even more. So if we look into the wattage output of panels we will find this is based on a STC value. Or Standard Test Conditions. We can argue if they are realistic, but at least it gives us a way to compare the expected output of panels. The STC for panels is generally 1000W/m˛ , Cell Temperature of 77°F and a Air Mass of 1.5.
All that is great but how does it help. Well I just added a Solar Irradiance Senor to my rig. A while back I found one of these on Ebay. I was lucky to get one with late enough firmware that it works with my Victron Cerbo.
I had seen these in the Cerbo Manual, but the cost of a new one was greater than I wanted to pay. So I was happy when one popped up on Ebay that would work. Most of the one's you see are not ModBus or if they are their to early of a firmware. (read that as can't update).
So I just got around to installing it on the van.
Well not I can tell you what two out of the three variables are Solar Irradiance , and Cell Temperature.
The above is data from the Victron VRM Data.
And for the fun of it I put it into my own program.
I will start tracking these numbers to see if I can put any predictability into this. The big chunk of the data is the Irradiance output. If I just take 28.9 percent of the available 190 watts, (two panels active) that give me around 54 watts, further derating do to temperature and air mass, or even dirty panels probably account for the lower 44 watts produced.
-greg