You are talking about the ones you attach to a wall, right? These are low wattage, so they work if you leave them on for a long time - they provide background heat. If you want something to raise the temp in a room by several degrees in a short time when you come back to a cold house after work, panel heaters won't do that job. They also say on the box what size room they are designed to heat and it's a relatively small room, so for a medium sized room you might need 2 or 3. Econoheat are SABS approved AFAIK.
All heaters work equally as the amount of energy required to heat a certain volume of air is constant. Different heaters just do it within different timeframes. As long as the heat generated is greater than the heat loss from the room it will work i.e. why you might need more than one depending on size of the room. Will just take a long long long long time