Yes, EPT means Enveloping Plasma Torpedo. Defensive means Shotgun.
If you increase the total energy requirement, then the torpedo is going to take longer to complete.
For example, the Plas-R has a total energy requirement of 9.0f. I get that number by multiplying the charging cost (3.0f) by the default amount of turns (3) needed to arm the torpedo. I don't know the location of that 3. I know it from playing the game, just as I know Photons take 2 turns to finish charging.
The charging icon looks at the amount of energy accumulated so far. It compares it to the total requirement. If it has 4.5 energy in the bank, then you still need to contribute another 4.5 energy. It will show as halfway there, 50%.
If I modify Plas-R to require a total investment of 18.0f energy, then it will take 6 turns to complete. You follow? Every turn it puts in a default of 3.0f. After 6 turns, it will have 18.0f energy.
So in this example, the charging cost hasn't been modified. The time needed to charge the torpedo has tripled doubled. The amount of energy invested has also tripled doubled. This is a bad situation for the War Eagle! The Plas-R's damage hasn't increased at all.
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New situation. If I lower the charge cost for the Plas-R from 3.0f to 1.0f, the War Eagle can charge a default torpedo (which needs the regular 9.0f of energy) at nearly top speed while cloaked. It will, however, take 9 turns to do so! (1.0f per turn * 9 turns = 9 energy invested). That's the trade off -- less energy per turn means more turns to finish the job. The icon will be at 11% after one turn, 22% after 2 turns, etc.
Clear as mud?
I know it's confusing.
[Edited for a mistake.]