Most solar power banks I've tested were gimmicks - this one is the real deal

"During peak sunlight, the Earth can receive as much as 100 watts per square foot at the top of the atmosphere. However, factors like clouds and rain and the angle of the sun over the horizon reduce this amount. On average, about 15-22 watts of energy falls on every square foot of Earth, which works out to 0.104-0.153 watts per square inch."

"Most of the ones I've tried in the past have been downright terrible, some even came with fake solar panels or solar panels that would actually drain the battery when out of the light. The others were mediocre, with the solar panel serving as little more than a gimmick to lure in buyers. But this one is different."

"A panel like this isn't going to collect buckets of energy, it's more like thimblefuls. That said, solar energy is free and available every day, so even small amounts can be useful in the right circumstances. The ROC power banks come in 16,000 mAh and 24,000 mAh capacities."

Solar energy reaching Earth's surface averages 15-22 watts per square foot after atmospheric losses from clouds and sun angle. While solar panels integrated into power banks collect modest amounts of energy, recent technological improvements make them practical rather than gimmicky. ToughTested's ROC series represents a significant advancement in solar power bank design. Previous solar power banks often featured fake panels or drained batteries when not in sunlight. The ROC series genuinely harnesses free solar energy daily, with capacities of 16,000 mAh and 24,000 mAh. Though solar panels on power banks collect energy in small quantities, consistent free availability makes them useful for supplemental charging in appropriate conditions.

Read at ZDNET