I have compared the 6.4 dm² panels with my 11.2 dm² under harsh conditions. The light intensity peaked at 4300 lux and it was impossible to distinguish the contour of the sun. I managed to charge close to 0.5Ah. I think at least 22 dm² ( 30W panel ) is necessary to do something meaningful in this kind of light. ( When it's snowing I've measured 170 lux )

The theoretically available power P at 4300 lux and a 11.2 dm² panel with 0.16% efficiency is:

P = 4300*0.000079*11.2*0.16 = 0.6 Watt ( I measured close to 0.4 Watt on the battery side )

Plotly from 30 december , comparison 6.4 dm² vs 11.2 dm²

Plotly from 31 december sun and clouds, 11.2 dm²