A pressure vessel balanced refuelling system enables greater volumes of gas to be used as working volumes and increases gas storage efficiency. The system includes a plurality of pressure vessels, each vessel having a liquid transfer opening for the entry and exit of a liquid that is used to displace a gas inside each vessel. A liquid transfer line extends from an outside to an inside of each vessel in the plurality of pressure vessels through the liquid transfer opening, and a liquid balance line inter-connects the liquid transfer opening of each vessel with the liquid transfer opening of each other vessel. Thus a balance liquid transfer path extends from an inside to an outside of one vessel, then from the outside to the inside of each other vessel through the liquid balance line, enabling a liquid level in the one vessel to remain approximately equal to a liquid level in each other vessel.