I found an old question asked 5 years ago. http://physics.stackexchange.com/questions/7260/how-much-power-to-keep-surface-of-aluminum-plate-at-given-temperature/262674#262674How much power to keep surface of aluminum plate at given temperature?. How do I answer this in a physics kind of way?
This is my original answer:
By running a current through one kilogram of metal using a adjustable D/C power supply 1 volt to 12v and slowly increasing the voltage until you read enough resistance to achieve the temperature desired. You can also get an adjustable amperage as well but may cost more. Also try using an after market defroster for back windows of cars. Just stick it on the underside. The plastic insulator should keep the metal safe to touch and will be more efficient. Like a light it is the resistance to the flow of electricity that gives off the light and heat.