The lightning storm will consume 10,000 EU per bolt, with a maximum bolt number of 10, and a maximum EU consumption of 1/10th of the maximum EU of the item installed in.
For example, a Lappack, with a max charge of 300,000 EU would produce 3 lightning bolts and comsume 30,000 EU each time. However, if you tried to use the overcharge module on an un-upgraded batpack, which has a max EU of 60,000, you would be unable to, as 1/10th of the maximum EU is only 6,000.
To fire an energy blast with the Discharge Upgrade:
4 Refined Iron
2 Advanced Circuit
2 Tesla Coil
1 Iridium Plate