How could I make a 1kW fan heater consume less electricity?
As has been covered in the other answers, you have your units confused. If you want to reduce the heat output from your heater to an average of 200 W, you need to pulse-width modulate the standard 1000 W output setting. To do this, you need a time-switch. On for 20 minutes, off for 80 - repeat. Finding a time-switch that allows you to set enough on-off times at the right precision might be hard - a temperature based approach might be simpler.

This will not reduce the peak power draw, or how hot the heater might get (to a first approximation). Switching like this will increase the chance that the heater fails sooner, maybe not by very much.