Yatiko, it depends on the type of heater, if an electrical load is placed on a generator greater than the driving power then the generator will slow down. Frequency and terminal voltage will drop, now depending on heater type, if they are purley resistive, they will just produce less heat. The turbine will not be damaged but depending on the generator type it may be damaged by overheating, through several factors probally too deep in theory for this discussion.
Have you thought of two stage heating which you can control ie 250 watts x 2.
Hope that sheds some light on your question.
allan down under