Methods and systems for concentrating oxygen include an oxygen concentration subsystem configured to generate a supply of oxygen-enriched gas, an oxygen delivery subsystem configured to communicate oxygen-enriched gas to a respiratory circuit for delivery to an airway of a subject, and one or more batteries configured to act as a sole power supply for the oxygen concentration subsystem and the oxygen delivery subsystem, wherein a ratio ROW is determined as: ROW=(O2 output)/total weight of the oxygen concentrator system, where O2 output is the maximum continuous oxygen output of the oxygen concentrator system, wherein the ratio Row is greater than about 0.19 lpm/lbs.
