I think that before getting too far off on tangents, just stick to the facts you have and don't add too many things into the mix. Those are that
1) Motor electrical power consumption = 746W per HP, plus losses,
2) The only information you have about the motor is the full load HP, you don't know what the actual load is on the motor.
But it's a relatively safe bet it's probably around 80-90% loaded, that's a good design practice. So since you don't know how loaded the motor is, but you also don't know the efficiency (losses), you can pretty much trade the unknown loading for the unknown losses and call it a wash.
So for NON-REGULATED energy charges, I would just go with the 15kW and if you want to charge for 1 hour of work, that's 15kWh. Keep it simple.
If you want to be gnat's ass accurate, buy a kWh meter.