For this second part of the charging observations I chose to use a USB 12V adapter as opposed to an inverter.
This particular 12V USB adapter is has an output rating of 5V and 2.0A which is 0.4A less than the output of the Apple 120V adapter. All this means is that it will take a bit longer to charge.
This one is branded Rayovac and I grabbed it at Wal*Mart to simulate what most boaters would use... The standard Apple USB to lightning cable was used with it.
NOTE: This USB insert-charger is a venerable POS. I can literally burn one out in a weekend. USB insert-chargers are one of the highest failing devices I see on boats. Go figure that most I see are el-cheapo Chinese crap purchased at checkout or at a convenience store.. Many of these USB insert-chargers are not designed to meet Apple's high current charging standard so will charge very, very slowly and usually default to 1A or less.
If you want a high quality long life USB insert, that actually works, consider the SCOSCHE USBC242M. This model, and it's 10W predecessor, have proven to be the most reliable USB inserts I have come across. The SCOSCHE USBC242M can charge two iPad Air's simultaneously at virtually the same speed as your 120V Apple wall charger. Amazon Prime usually has them for $11.50....