Q: Converting Wattage to Temperature Let us say you have an 8 watt source or heater, what is the highest temperature it can attain or be converted to?   
I was asking in the context of transformer losses. At no-load, there is core loss.. if the temperature of the core is 50 Celsius. what minimum wattage of the iron core loss can produce that?

A: The temperature will rise until it's dissipating heat at the rate heat energy is being put into it. If it were so well insulated that the temperature would have to be 20,000K for it to dissipate at  that rate, then that is the temperature it would rise to!

A: The temperature of an object is related to the total internal energy.  And you've supplied one thing that will affect the total energy of this item: the fact that there is a power input of 8 watts (8 joules per second).
Unfortunately, this doesn't tell you anything about the other flows of energy (heat loss).  If the object can easily lose that energy, then the temperature will remain very close to ambient.  If well insulated, the temperature will need to rise a lot to force the energy out.
An analogy would be to learn that a constructed pool will be pumped with water at a rate of 8 cubic meters per second, and asked how high the water will reach.  The answer depends on how the pool is constructed and how it holds water.
Likewise, the steady-state temperature of an object with a known power delivered to it depends on the object and its surroundings which control how the heat is removed.
