Hello. I am doing a project - leds around the house. Because I have 3W led lights I decided to put 440R resistor before. Now I have a problem because I need to calculate what wattage resistors do I need.
I dont know what informations do you need so:
At 12V - around 1A
At 7.9V - 9mA
And 12V with 440R resistor 9mA
(Tested with lab psu)
What wattage of resistor do I need to produce lower temperature on resistor because It will be placed on not so fire resistant place.
might be of help?
What is that resistor for? Is that for current limiting for a LED or LEDs? If so, a constant current supply might be called for to control the current, instead.
P = V * I
V = I * R
from that
P = I * I * R
or
P = V * V / R
That Ohm's Law Calculator didn't work for me but this one did:
9 mA across 440 Ohms dissipates 0.03564 Watts
A 1/4-watt or even 1/8-watt resistor will be fine.
Thanks for posting a reliable source of info that the OP can use to help himself.
I doubt very much that at such a current the 3W LED will glow.
Suppose that the OP measured the current incorrectly and he needs a completely different resistor
Whithout a link to the "3W LED", it' s shot in the dark.
Not if the LED is on 
@sterretje - go sit in a corner and think about what you id!
You are putting the LEDs in series combination to reduce the resistor voltage drop, right?
This calls out for a schematic. Please provide one, even if it's just a picture of pen scratchings on an envelope.
Dark corner? Or my I have the LED on?
Yeah... SOMETHING IS WRONG HERE...
0.009 Amps X 12V = 0.108 Watts.
Assuming your power supply is 12V that's 1/10th of a watt total for both the resistor and LED. A regular resistor would be fine but then you don't need a 3W LED.
Usually, we use a current limiting resistor for "regular little LEDs". Higher-power LEDs normally use a special constant-current switchmode power supply.
You need "extra voltage" for the resistor. With a regular little LED and resistor we might have an LED rated for 2V and a 5V power supply. That leaves 3V across the resistor and we can use Ohm's Law to calculate the resistance for the desired current. With more voltage across the resistor than across the LED, the resistor is wasting more power than the LED. This is inefficient but it's only a few milliwatts.
I think we need a link to the LED specs. But I'll make some more assumptions and you can use this as an example -
...Please double-check my calculations because I did this quickly.
Assuming your power supply is 12V
Assuming the LED requires 7.9V
3W / 7.9V = 380mA
That leaves 4.1V across the resistor. 4.1V / 0.380A = 10.8 Ohms so you can use an 11 Ohm resistor (or some "close value" whatever you can find).
4.1V x 0.38A = 1.6 Watts so you can use a 2W (or greater resistor).
And, your LED probably needs a heatsink. Check the datasheet and the manufacturer's recommendations. (If you run it at 9mA do don't need a heatsink. 
)
Bad idea to use resistors for 3watt power LEDs.
Power LEDs should be driven by a constant current LED driver.
This one can control/dim three strings of three 3watt LEDs (3x3) on a 12volt supply.
Leo..
If led Is connected on 12v its working on 3W.
But if its connected on 7.9v it needs only 9mA to work (I tested it with lab psu and multimeter).
Oh sorry I did a mistake on 9V it works on 3W
On 7.9V it works on 9mA - 0.07W.
(I will have 12V power supply)
I will have leds connected in paralel and before every led I will put a resistor.
And I want to put a "fuse" (dc circuit breaker/din rail automatic fuse...) and I would need 12V 2A circuit breaker but I cant find one (I will have some motion sensors connected on psu so I will use 2A psu).. can I use 24V circuit breaker? Or do you know or have any link?
Does your LED come with any specs?
Is it one LED or is it 3LED's in series in one thingy?
What do you mean with it works at 9mA?
It glows? Why buy a 3W LED if you want a glow?
Led doesent come with any specs.
Led driver that comes with led is 9-16V 0.2A
In this led enclosure is about 10cm long led strip.
I was testing led with my laboratory power supply and I got right brightness at 7.9V an current draw was 9mA.
Yes it glows. It starts glowing at about 6V.
Because I have 3W led because I didnt use them for something else.
Guess it has 3LED' s in series.
If you want to feed it from 9V with 9mA, you will need:
R=(9V-6V)/0.009A =3000/9 = 330 ohm.
Not at all any need for constant current sources with transistors if you want it at this brightness.
It will use 60 mW in the LED and 30mW in the resistor.
Fuse not needed...