I'm not talking about physical size. I'm talking about cost and complexity. An air tank with a compressor pump at one end and a turbine at the other is simple, reliable, well-understood, off-the-shelf technology. How much energy can you store in an air tank that would fit on the back of a truck? Hook it up to another truck full of solar panels, and you have a local electric grid, capable of powering at least a few buildings, at very reasonable cost and minimal sophistication required for operation and maintenance.