mythbusting: solar makes zero watts in cloudy weather

As with most claims, overstatement limits usefulness. Telling newbies that solar makes zero watts in cloudy weather can cause them to make choices that do not fit their use case.

example

I am in 100% overcast conditions (see pics above), and at the time I took the measurements the flat 750w array was making 212w. Correcting for the height of the sun right here/now (see below) this is ~51% of what the array would make in lab conditions. Under field conditions with clear skies my system averages 83% efficiency after cell temp, wiring, and MPPT losses.

the math

It was ~1130 local when I took the measurements, which is 50 minutes away from solar noon. The cosine of solar zenith angle¹ tells us that irradience is .6633 of lab standard irradience so I would expect 663.3 watts/sq meter (1,000w/msqft STC x .6663) to land on the panel.

After system losses of ~17% (see above) in clear weather it could/should be pulling 412w. 212/412 = ~51%.

addendum

Finishing this post at 1310, well past solar noon. Harvest is now ~52% of what it would make under clear skies. And it’s harvested 1.16kWh so far, clearing my 1.0kWh minimum requirement to run the van. I prefer to get closer to 2kWh/day but the cloud cover is being stubborn. The forecast suggests it might only be partly cloudy by 1600, but by that time there’s not a lot for flat panels to capture.
After today the next 10 days are forecast to be partly cloudy or better. Because of overpaneling I should be able to use power like normal on those days. If I get desperate I could put out the 200w panels. I don’t see a reason to do it but it might actually inspire me to install the port for plugging in the panels to the exterior of the van…

update from dusk

The sun never did come out. Ended up getting 1.63kWh today before panel voltage collapsed near sundown, pretty close to my historical average consumption of 1.83kWh/day. This is why we boondockers overpanel…

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1. I used the Solar Tracker app for this and for solar noon numbers, but it’s also calculable with online tools or raw math. :-) ↩