I cannot directly address your question concerning a 500 watt heater but can concerning the 750 - 1500 watt plug ins.  We usually are not in a position to use electricity as we usually boondock but for those occasions where we have it, the 750 watt option works quite well as long at the temps are 40 and above.  When using the 1500 watt option, after 5 minutes or so the breaker will pop and you will have to re-set it. 

If and when you get one of these make sure it has the safety button underneath so that in the case IF it gets tipped on its side it will automatically turn itself off.