How to decide how many ohms for a heating resistor?

[errjie]

I'm doing a project that need to use heating resistor as a heater. I need to maintain a 37 Celsius in a 25 cm3 box. I plan use 6 ceramic 5W resistor to heat up the space temperature and supply is 12V DC, but how to decide how many ohms i should use?

 

[shortbus=]

Maybe this will help; https://www.mhi-inc.com/Converter/watt_calculator.htm

 

[BeerBelly]

P=E*I Power=Volts x Amps
E=I*R Volts=Amps x Ohms
P=E*(E/R)
PR=E^2
R=144/5
R=28.8
I didn't double check my work, but a 5 watt resistor should be no less than 28.8 ohms at 12 volts.

 

[MikeMl]

I'm doing a project that need to use heating resistor as a heater. I need to maintain a 37 Celsius in a 25 cm3 box. I plan use 6 ceramic 5W resistor to heat up the space temperature and supply is 12V DC, but how to decide how many ohms i should use?

Do you know for a fact that you need 6x5W = 30W to maintain the temperature in the box when the outside is at the lowest temperature you will encounter?