Is the Wattage rating on resistors a maximum input rating or the output wattage?

Posted on 2016-02-18 19:06:00 in Engineering by WILL

For example, say I have a 9 volt battery and want to add a resistor to supply only 3 amps to my load. I know I need a 3 Ohm resistor, but something that won't burn out at 27 watts. I can either buy a 3 Ohm 27 Watt resistor so the resistor's wattage equals my circuit's wattage, or I can buy a 3 Ohm 50 Watt resistor so there's some wiggle-room. 

Would I want to buy the resistor whose wattage matches my circuit, or is the wattage rating just a maximum and I should buy something of higher wattage?

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