The usual cheap RO systems operate from household water pressure only. They fill a pressure tank at the output, allowing the purified water to be dispensed from a faucet. The pressure across the membrane is that input pressure minus the tank pressure. That's relatively low, so the efficiency is poor, wasting perhaps four gallons for every gallon of pure water.

You can improve the efficiency with an electric booster pump, or with a "permeate pump" that recovers energy from the exiting waste water, or by simply filling a pitcher with no pressure tank. I'd expect that even the most wasteful systems are still a small share of a typical household's total water consumption though, assuming they're used only for drinking and cooking water.