In order to calculate the wattage we need to know the voltage and the resistance and Ohm's law.Also we need the formula for power
which is P(power)=V(voltage) * I(current)
As we see we already have the voltage, so we need to find the current. From Ohm's law R(resistance)=V(voltage) / I(current)
we transform the formula to I =V / R
so the power formula becomes
P=(V *V) / R
After we found the power that the resistor requires we have to find a resistor that will handle that power.
And also temperature will degrade the capacity of the resistor to handle power.For example a thin film resistor will handle 80% of power dissipation at 85 degrees Celsius and 50% at 100 degrees Celsius.
So it would be advisable to add a lot of margin . So having a resistor with at least double the wattage required would be advisable.
One problem with having resistors with bigger power dissipation is their size.Increasing the size of the resistor will increase the size of the board which will increase the price.
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