I recently built a portable generator using a 100Ah AGM battery and peripherals. You can read all about it here on my web site
To charge it I bought a 120w folding power panel
I used a 150w inverter to charge my new laptop and drew the battery down to 80%
Up here in New England, the sun is very low on the horizon. I set the panel up on my dark, wood deck and to get good sun I had the panel nearly vertical with just a few degrees tilted back. I was seeing 4.8A with the sun being filtered through a few leafless trees.
Seeing my yard covered in snow I tried an experiment. Moved the panel off the deck and into the snow.
with the reflectivity of the snow I saw an increase of an average of 1A
BTW…By charging from 9am to 4pm my battery was back up to 98%……Not bad considering the suns height, length of charge and tree branch filtering. Should be great this summer or when I go to New Mexico in January