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You can find the resistance of an incand. bulb at various voltages by deriving other formulas from the P = V^0.6 relationship given by Wiki.
The intensity varies by V^3.5 but the intensity perceived by the eye is yet another thing.
As I mentioned in another thread, Jeep and Chrysler cars in North America operate the high beams at FULL POWER as daytime running lights. Only the lights and the eyes of oncoming drivers get hot, especially when the weather is cloudy.
I agree with Panic Mode that most cars dim the low beams with PWM as daytime running lights and since they are dimmed then they last for a very long time.
My Chevrolet car used the turn signal lights as daytime running lights but I didn't notice if they were dimmed. I DID replace a burned out light bulb.
Some modern 25W resistors have an aluminum finned attachment.
Strictly speaking it is not possible to know the power rating of a larger power resistor without obtaining the data sheet. That's because they are rated not only by power alone but by maximum temperature rise. This means that one manufacturer might rate theirs at 20 watts 300 degrees C max, and another at 20 watts 250 degrees C max, and they are both the same size (or nearly so). As misterT points out you can compare sizes to some degree but be aware that they dont necessarily have to follow the size relationship rule.
So the first line of action is to find the data sheet from the part number hopefully on the side of the resistor. If it is not found, then you could try testing it. I dont think they make any ceramic resistors that are rated below 200 degrees C (correct me if you find one that is) so you could use that as a general guide. Using a power supply and an IR temperature gun, run the resistor at some voltage that generates some heat then measure the temperature after some time. If the temperature is below 200 degrees C, increase the voltage a little, let run for a time, then measure the temperature again. Eventually the resistor will get up to 200 degrees C and then you can calculate the power in the resistor from P=V^2/R. That will give you an estimate.
I'd like to say go up to 300 degrees C, but if you do that and it passes the 'test' that doesnt mean it will be reliable. If the resistor was not made to take that high of a temperature it could fail much sooner than it should. Obviously if it starts to smell bad you should stop the test
But there are a lot of pictures available online at places that sell resistors too, so you could compare as a start if you cant find the data sheet. If you find one that looks exactly like the other it might be a good bet it's made by the same company.
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