https://sierramadreresearch.zendesk.com/hc/en-us/articles/4404474634893-How-much-should-a-UL-battery-charge-with-two-panels-over-4-hours-in-full-sun-
The panels need direct sunlight, no windows. 4-5 hours of direct sunlight should give about a 2 bar charge on the UL battery.