I started using an Inogen One G5 about 3 months ago. I purchased the unit outright from Inogen as since this is a long-term need, renting equipment made no sense financially. I purchased one single and two double batteries. The single is great for lighter activities as it weighs less. I got the two doubles to meet airline requirements for longer flights. Though please note that I just got back from England and the AA gate attendants at both O'Hare and Heathrow had no interest in checking my battery reserves.

Also.. you may want to check the 1-6 settings on this unit. My understanding from someone in the industry is that these are just machine settings, not liter settings. But I have not yet explored this further myself.