System for producing an oxygen-rich gas comprising (a) a primary gas mover including a first and a second compressor, wherein the primary gas mover is characterized by a weight W.sub.p; (b) a drive motor adapted to drive the first and second compressors; (c) a rechargeable power supply characterized by a weight, W.sub.b; and (d) a pressure/vacuum swing adsorption unit adapted to separate the pressurized feed air into an oxygen-rich product at a product flow rate F.sub.p and an oxygen-depleted waste gas, wherein the adsorption unit comprises a plurality of adsorber beds containing an adsorbent and characterized by a total adsorbent weight W.sub.a; and wherein the combined weight, W.sub.t, of the adsorbent, the primary gas mover, and the rechargeable power supply is characterized by the expression 0.75 F.sub.p