A heater will output heat no matter what wattage you put into it. But if you're only putting 5 watts of power into it, you won't get much heat out of it. If you're putting 500W into it, you'll get a pretty good bunch of heat out of it.
Heaters are like light bulbs, the more voltage and wattage you put into them, the more you'll get out. With a bulb, it's light. With a heater, it's heat.
Hope that helps and didn't sound condescending. It wasn't meant to.