We should think about ruggedness and reliability as two different metrics. Some models are quite good at dealing with physical impacts, vibrations, moisture, dust, etc. That doesn’t always translate to reliability for ordinary use, or failures caused by other things related to design or manufacturing, or even software/firmware issues.

The theoretical maximum would be whatever kinetic energy actually hits the panel.

If a 2mm diameter raindrop has a mass of 0.034 g and has a terminal velocity of 9 m/s, the total kinetic energy is 0.00275 joules per raindrop.

The threshold for what is considered heavy rain is about .75 cm of rain per hour, so for our square centimeter panel we’d be talking about .75ml of rain per hour, or 22 of those average sized rain drops per hour. That’s only .06 joules per square cm, over an hour. That’s 0.000016 watts.

2 square meters is 20,000 square cm, so that’s .33 watts on the 2 square meter panel. Not significant enough, even on heavy rain.