[kragen]

The panel voltage is pretty stable until the illuminance gets really low (unless you're drawing a lot of current). Diodes such as solar cells are roughly constant-voltage devices. You can get a pretty long way avoiding filesystem corruption by mounting things read-only, but (I've heard) some SSDs aren't really read-only even when they're read-only, because of read disturb and the attempt to compensate for it in the FTL. 10 seconds of 2 W at 3–6 V is about two farads, so you might be able to get acceptable stability with a supercapacitor in the 1–10 farad range instead of a battery.

[lightedman]

"The panel voltage is pretty stable until the illuminance gets really low"

I'd like to see what panels those are, because the ones I've built while working as a PV manufacturing tech, both mono and poly (roughly 21% efficiency,) will have greatly varying voltages with even the tiniest hint of cloud cover over one cell, even with the junction box working to help separate out sections of the panel to maintain better voltages. Typical 60 cell 30-32V panel will drop to ~18-20 with just two cells on one 20-cell section of the panel covered. Sure this is still enough for the paltry voltage this specific server needs, but if they used smaller and more affordable panels like those used for cell phone chargers or similar size (within about 18"x18" form factor,) I can guarantee you those do not take to shading or even bad orientation well at all. 45 degrees off direct-exposure and you could be looking at that smaller panel producing a mere 2V or less.

[kragen]

Is that the MPPT voltage or the open-circuit voltage? I was thinking of a near-open-circuit voltage (which is what you have if you're powering a 2-watt webserver from a 50-watt solar panel), but MPPT will vary a lot more. Also, covering 6% of your cells will drop your voltage a lot more than covering all your cells 6%.

[lightedman]

Open circuit. This is one specific behavior we looked out for when testing panels before shipping, after the EL test, lamination, and junction box installation.

[kragen]

Huh, I guess I must be totally wrong, then. Thank you for the generous correction!

[lightedman]

You're welcome. Even without MPPT inverters/chargers in the mix, you can take a 1,000V (maximum US NEC allowed IIRC) string of panels (obviously in series) fully black out one cell out of hundreds, and your entire system will do nothing (assuming your panels do not have bypass junction boxes and are straight-series.) That blacked out cell essentially acts like a full clog in a pipe - nothing will pass because of that 'dead' cell, until you remove that whole panel and replace it or bypass it.

We test by shading different cells at a known light level, and seeing how far the panel as a whole deviates from our expected baseline. Usually what we'll find is that a cell hadn't had its ribbon bars properly soldered while in the tab/string machine, so the shading of the cell dramatically reduces performance. You can't catch this kind of defect easily in an EL tester, as usually you've got enough contact between ribbon and cell to get the entirety of the cell to emit light, thus hiding defects from soldering. So we implemented that shading test to root out bad cells/bad soldering jobs. Quite proud of having thought of that testing methodology, because I've not seen any other company test panels in that manner.

[driverdan]

That's a reasonable point. So long as the panel's output voltage is higher than the controller's minimum required input it would be stable.

I still wouldn't want to run a Pi directly off solar. I'd want enough warning to shut it down rather than killing the power.