Finding the wattage rating of resistors and variable resistors?

Is there a way to find the wattage rating of resistors and such mathematically? or is it something that has to be tested in real life? I have quite a few resistors that I'd like to use, but don't know the wattage rating of them. I only know their resistance, and I could calculate other values if I put voltage across them with a multimeter. Also, what type of variable resistor would you recommend to control a 12 VDC device that draws 12 amps?

Regarding the math, the power dissipated by a resistor flows out of its surface as heat. So the amount of power a resistor can dissipate (without getting so hot it burns itself up) tends to increase with its surface area, which increases with its size. Basically what Iceng said.

A variable resistor to put in series with a 12 VDC source, and a load that maybe looks like a 1 ohm resistor (draws 12 amps at 12 volts). It turns out that the most resistor placed in series with that 1 ohm resistor, which would draw the most power, is, another 1 ohm resistor. As Iceng has already shown, the amount of power dissipated by that 1 ohm resistance would be 36 W.

That is indeed a beefy variable resistor. I think when they get to that power rating, they start calling them "rheostats". A variable resistor able to dissipate that much power will probably look something like this: http://en.wikipedia.org/wiki/File:Pot1.jpg

Most circuit resistor wattage can be determined by the size ( volume ).

Bigger resistors have the wattage screened on surface near the ohmic value.

12 VDC at 12 Amps controlled by a a resistor. Since you don't specify, I select the resistor has to be able to vary the voltage to your unknown load to 6 VDC, further this load is resistive and will only draw 6 Amps at the lower voltage.

This variable Resistor = 6 V / 6 A = 1 ohm maximum. Resistor_{power =} V × I = 6 × 6 = 36 Watt

http://www.instructables.com/answers/How-can-you-tell-or-measure-how-many-watts-a-resis/

has my attempts to answer this question.

Regarding the math, the power dissipated by a resistor flows out of its surface as heat. So the amount of power a resistor can dissipate (without getting so hot it burns itself up) tends to increase with its surface area, which increases with its size. Basically what Iceng said.

A variable resistor to put in series with a 12 VDC source, and a load that maybe looks like a 1 ohm resistor (draws 12 amps at 12 volts). It turns out that the most resistor placed in series with that 1 ohm resistor, which would draw the most power, is, another 1 ohm resistor. As Iceng has already shown, the amount of power dissipated by that 1 ohm resistance would be 36 W.

That is indeed a beefy variable resistor. I think when they get to that power rating, they start calling them "rheostats". A variable resistor able to dissipate that much power will probably look something like this:

http://en.wikipedia.org/wiki/File:Pot1.jpg

If your looking to control the voltage you'll need more then a variable resistor.

Bigger resistors have the wattage screened on surface near the

ohmic value.

12 VDC at 12 Amps controlled by a a resistor. Since you don't specify,

I select the resistor has to be able to vary the voltage to your unknown

load to 6 VDC, further this load is resistive and will only draw 6 Amps

at the lower voltage.

This variable Resistor = 6 V / 6 A = 1 ohm maximum.

Resistor

_{power =}V × I = 6 × 6 = 36 WattA