Thanks for the replies.
I am using a standard emersion heater in a domestic water tank, so I would like to avoid non standard elements. The idea of using multiple elements is interesting, however I think I would struggle to get the fine control that is needed for charging the battery. I am also not keen on bringing high current wires into the house.
The inverter is also used to power the lighs and computers/tv, switching for longer periods of time would probably cause flickering so I would like to avoid that.
Changing the PWM frequency is definitely an option, so I will take a look at doing that.
Does anyone have any sugessgions for a filter design with the PWM at a higher frequency?
As for the last point "Has anybody pointed out that switching a heater on/off at 10kHz is silly...?", in what way would you sggest modifying the program?
Thanks for your time.
Tom