ON THIS WIKI
The Multi Functional Electric Storage unit or MFE is the second tier of energy storage devices in IndustrialCraft 2. One MFE is capable of storing up to 600,000 EU making it the equivalent of 15 BatBoxes. The MFE can accept a maximum power of 128 EU from any of its 5 input faces and emits a current of 128 EU/t from its output face (the face with the dot.) This current will destroy all first tier machines if it is not first transformed. The MFE will not explode if it reaches full capacity.
The MFE can also accept a redstone input. This will change the action of the MFE depending on which setting is selected. This setting can be changed by clicking the button with an image of redstone on the top right. Attach a lever to a MFE can be done by "sneaking" while right clicking on it.
Aside from storing EU, it also acts as a connection between the main power grid and some GregTech machines such as the Industrial Blast Furnace and Industrial Grinder which need 128 EU per tick since these machines don't have their own internal power storage.
Here is a video geared towards intermediate and advanced players that will explain some of the more advanced concepts of IndustrialCraft EU power.