Space heater trips inverter breaker (R-Pod 195)

The current output of solar cells/modules that is linear with solar irradiance (photons to electrons) is the short circuit current (Isc) or current at zero voltage. If the fill factor (how close to square the current-voltage curve of the module is) is poor then the current at max power (Imp) around 17V for our 12V modules can drop off significantly from Isc. Good modules will have a nice flat upper horizontal part of the IV curve with very little drop off.

So if your SunWize modules had issues (like mismatched or broken cells or interconnects) then you could have a significant drop off in Imp at partial sun levels. Having multiple mismatched modules in series can make this worse too. The SunWize array was also probably not performing very well at full sun, it was just not as noticeable.  But for new modules at 12V in our little systems it's not something we need to really worry about. 

The purpose of a lot of marketing is to instill fear of making a mistake into consumers. This is especially true for relatively mature commodity type products, like say flashlight batteries or solar modules. There is really little difference between them so the companies' marketing folks try to come up with stuff to make you afraid to buy the other guy's product. Since they all do that the consumer often winds up confused and afraid of buying anything which hurts everyone's sales. 

Sorry to give the impression that you have to have full sun to get output from a PV module. Being a professional in this technology I tend to gloss over things sometimes.  The topic I was adressing was differences between the various solar module manufacturer's low light performance, which is negligible. 

The output current of solar modules is essentially linear with solar irradiance.  If you have a very cloudy day you might get about 10% of one sun. A day with high cirrus cloud cover might be 50-80%. So under very cloudy conditions a solar module will produce about 10% of the current that it produces under full sun conditions. That is because one photon of sunlight produces one electron-hole pair in the silicon which the solar cell is fabricated from. Nothing you can do about that, you can't collect energy that isn't there. 

One trick you can do on a very cloudy day is lay the module horizontally rather than point it towards the sun. if you can't tell where the sun is the module can't either so its better to lay it flat so it can collect sunlight scattered around the whole sky by the clouds.  

Re the positive vs. negative ground issue, I think you are perhaps referring to the surface polarization problem Sunpower's early products had in high voltage negtive grounded grid tie applications. The module frames must be grounded on these large high voltage systems for safety and when you do that it imposes an electric field between the glass surface and the top surface of the solar cells. In Sunpower's case they depended on what is effectively a static charge on the front cell surface to achieve their really high cell efficiencies and the electric field from negative grounding bled that off. Sunpower is far from being a cheap product though. Other manufacturer's might have had this issue too but I haven't heard about it. 

The surface passivation issue wouldn't effect our little 12V systems because the voltage is so low and we're not grounding the frames on these portable modules anyway. Besides, Sunpower doen't make their products the same way now as they did in the 00's so its no longer a problem for them even in high voltage negatively grounded systems. That doesn't stop other companies' marketing ppl from making stuff up of course... (disclaimer, I've never worked for Sunpower or owned their stock )