Hi,
To go a little farther, if you do want to spec wire in terms of wattage then
you have to specify the operating voltage such as 12v, 120v, 240v, etc.
You can then divide the power (watts) by the voltage and get the required
amps, then look up a wire that can do the amps.
For example, say you have 1200 watt heater, and your home voltage is
120vac. Divide 1200 by 120 and we get 10 amps, so we look up a wire that
can handle 10 amps, at least. 15 amps even better.
On the other hand if your heater is 1200 watts and your home voltage is
240vac then divide 1200 by 240 and we get 5 amps, so you would then
need a wire that can handle at least 5 amps (10 amps even better).
You can always go higher in amp rating too, as long as you dont mind
the heavier wire.