I ran simple experiment. First, I built simple set: Generator -> MFE -> LVTF -> BatBox -> Macerator. Then, I put some fuel into generator: BatBox got full (40000 EU) charge, MFE got 66664 EU.
Then:
- disconnected BatBox from LVTF,
- put 1 Iron Ore into Macerator, waited until it's done:
BatBox charge - 39213 EU (-787 EU per operation, okay)
- connected BatBox back to LVTF:
BatBox charge - full (40000, +787 EU)
MFE charge - 65896 (-768 EU, weird already)
- put another 1 Iron Ore into Macerator, waited:
BatBox charge - full (40000)
MFE charge - 65896 (-896 EU per operation, umm...)
Next stage:
- disconnected BatBox from LVTF again,
- replaced Macerator with Extractor,
- put 1 H. Coal Sell into Extractor, waited:
BatBox charge - 39213 EU (-787 EU per operation, okay)
- connected BatBox back to LVTF:
BatBox charge - full (40000, +787 EU)
MFE charge - 64232 EU (-768 EU, er...)
- put another 1 H. Coal Sell into Extractor, waited:
BatBox charge - full (40000)
MFE charge - 63464 (-768 EU per operation, *pokerface*)
- disconnected BatBox from LVTF again,
- put 2 H. Coal Sell into Extractor, waited:
BatBox charge - 38413 EU (-1587 EU total, -793,5 EU per operation, what the he...)
- connected BatBox back to LVTF:
BatBox charge - full (40000, +1587 EU)
MFE charge - 61800 (-1664 EU, WHAT THA!..)
This entire story leaves me only one question: what the heck is going on? 
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