Invertors only "use" as much as you're drawing. (plus a little overhead to make the invertor work) It -WILL- drain your battery overnight if left on, as is the nature of most car appliances. However, the 400w Continuous is simply referring to the maximum output that the invertor can sustain. that particular invertor you're using could power, oh say... a 13" television -and- dvd player?
If it was cheap and you can't return it, I say stick with it.