Although the on-board charger is rated at a maximum of 7kW per phase, it will charge at a lower current. EVs normally come with a 13A three pin plug in case a dedicated charger is not available, and most will charge down to about 6A.
Charging from your 2.5kW generator would only add about 10mrph (miles range per hour charging), compared to 90 odd on a fast charger, or 400mrph the Tesla Supercharger network chucks out.
