Wonder why your solar output isn't what you expect?

[Scalf77]

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/m² , 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

[boywonder]

...just start at NOCT instead of STC.......you'll feel better......

[jonyjoe101]

I found out through trial and error the main culprit is voltage drop from the solar controller to the battery. Every single controller I owned mppt and pwm had voltage drop.

For some reason they all read at least a .5 volt high. Which means the battery is never fully charged. Increasing or decreasing the bulk setting just .2 volts up or down will make a difference on how much amps are going to the battery.
Its rare that a controller has voltage calibration, I only seen it on the makeskyblue 60a mppt.

I suspect if the controller is properly calibrated you will see the panels max output when the sun is overhead. Which is also the hottest part of the day.

On my 365 watt panel with a makeskyblue 60 amp mppt, when the sun was overhead. I was able to actually get 375 watts, a little more than the max rating.


On the smartsolar charger mppt picture, if 17 volts is the PV actual voltage, its too low for mppt to extract any extra amps out of it. With 190 watts of panels you should be able to get at least 9 amps of charge power when the sun is overhead, but only if the PV voltage is more than 30 volts and the controller and battery terminals are reading the same.