An energy storage unit with capacity determined by the battery/crystal items placed in it.
In my perusal and brutalizing editing of the wiki, I came across the MFSU page and noted, with some dismay, the line: 'This is the only storage unit, out of 4, that stores less energy than the sum of its component [sic] does.' (Which is true - 40M for the MFSU vs 64M from 6 lapotron crystals and the MFE.) So I thought about that, and a fittingly IC2 sort of way to address the issue, and came up with the idea of the EESA, or Expandable Energy Storage Array.
Long story short, it's a box with some number of slots in it, in a grid - maybe 3x3 or 4x2 or 4x4, it doesn't matter as far as the concept is concerned - into which you place RE Batteries, Advanced RE Batteries, Energy Crystals, or Lapotron Crystals (or maybe something else, like the charging versions). Essentially, the box would wire them in series and act as one giant energy storage box, discharging and recharging the individual items instead of a single internal reservoir as in the current EU storage blocks. Thus, you could expand your storage a little more gradually as you acquired the resources to do so. I've been doing some research on batteries and what might make sense somewhere between IC2 and reality, and this is what I came up with.