[Letum]

I don't know how simple I can make this.

You cant get energy out of nowhere.

If a disel motor used 1 unit of energy to go 1 mile that means a electric motor also uses 1 unit of energy to go one mile because it is useing the same screw to move the same boat.

When the diesel motor uses 1 unit of energy it uses 1 unit of fuel.
When the e-motor uses 1 unit of energy it uses 1 unit of battery power.

Then 1 unit of battry power needs to be replaced when the battrys are charged.
If battery chargeing efficany was 100% then it would take 1 unit of energy to charge it.
The disel motor can give the battry 1 unit of energy. We allredy know that When the diesel motor uses 1 unit of energy it uses 1 unit of fuel.
So at 100% battery chargeing efficancy it takes 1 unit of fuel to charge 1 unit of battery power.
However battery chargeing efficany is less than 100% so more than 1 unit of fuel is needed to charge 1 unit of battery power.

Stock SH3 works this way. Any mod that changes it is VERY unrealistic. Exploiting this is also Very unrealistic.