Sizing generator to charge batteries

Putting in 100W of solar costs can cost less than a set of GC2s if you have any handyman skills at all. Add $100 for an upgrade to a higher quality name brand controller. And, doing so will not only extend stays when off-grid, but will lengthen the life of the battery overall by ensuring they're properly topped/maintained even when in storage (unless you store inside, I suppose). People spend the same amount for a PD or Iota converter to replace a cheap WFCO one, when a solar setup would provide more value, even on-grid. Let the WFCO handle bulk charging and cloudy days - the solar will still supplement and top up the battery on a regular basis.