When it comes to electric heat, all plug-in heaters deliver identical efficiency, regardless of what advertising promises to the contrary. The process of converting electrical energy to heat energy is 100% efficient, so there’s no significant difference as far as operating costs or the environment. The only real issue is what the heaters look like and how convenient they are to operate. I’ve had good results with a type of heater that’s reminiscent of an old-fashioned rad. In the days when I heated my office loft electrically, one 1500 watt heater was enough to keep the 500 sq.ft. space warm. The coldest days sometimes needed a second heater if it was windy. The only drawback is that this model makes a kind of clicking noise when it first gets warm from a cold start. Cost of operating a heater of this size works out to 22.5 cents per hour based on the 15 cents per kilowatt hour that my all-in electrical bill typically comes in at. This takes into account the fact that about two-thirds of my bill is charged at off-peak rates. Assuming the heater operates 50% of the time, it costs about $2.75/day to run a 1500W heater, or about $80/month.