So, the panels get bigger and heavier of course as the power increases. I like the idea of the 40w panel as it is easily stowed and light weight. Could a 40w panel re-charge a group 31 AGM (half depleted) in a day with a decent amount of sun?
NO..not enough power
a grp 27 at half charge is down about 50 amp hours
the 40w is going to charge at Maybe about 2.5 amps , it would take over 20hrs to
charge to full
it would extend your battery use during the summer
it would not do much to help you in cold weather, with the furnace use at night
"Any Charging" is a help,
but consider at least 100w
this would give you about 5.3amps charging
if your a weekend camper
and start out with a fully charged battery,
this will help you thru the weekend
but you need to fully recharge the battery when you get home from camping