Premium Membership ♕

Save 50% on all EEP Academy courses with Enterprise Membership Plan and study specialized LV/MV/HV technical articles & guides.

Home / Technical Articles / A Look at the Power Rating of Resistors
Figure 1 - Common form of resistor
Figure 1 - Common form of resistor

You may be asked: “what is the power rating of the resistors you want to buy?” when buying a resistor to build a certain circuit. For the most standard class of resistors, you may simply be given ¼ Watt resistor.

The power rating of a resistor is the specification given with a resistor that serves to tell the maximum amount of power that the resistor can withstand.

Thus, if a resistor has a power rating of ¼ Watts, ¼ Watts is the maximum amount of power that should be fed into the resistor.

When an electrical current passes through a resistor, electrical energy is lost by the resistor in the form of heat and the greater this current flow, the hotter the resistor will get. Heat is generated as the current passes through electrical components. The heat is usually negligible and unnoticed in a circuit if the current is small enough and suitable for the circuit. A substantial amount of heat in a circuit can be created if the current is large enough.

The reason why resistors are given power ratings is because current can melt components and possibly create shorts in a circuit if the maximum allowable amount of power that can pass through the resistor is not specified.

Figure 2 - Resistor ratings
Figure 2 - Resistor ratings

The Resistor Power rating is sometimes called the Resistors Wattage Rating and is defined as the amount of heat that a resistive element can dissipate for an indefinite period of time without degrading its performance. Depending upon the size, construction, and ambient operating temperature, the power rating of resistors varies a lot from less than one tenth of a watt to many hundreds of watts. For an ambient temperature of +70 degrees Celsius, most resistors have their maximum resistive power rating given.

Since the standard power ratings of 0.25W or 0.5W are suitable for most circuits, the power ratings of resistors are rarely quoted in parts lists. It should be clearly specified in the parts list for the rare cases where a higher power is required.

The Resistor Power Triangle

From Ohm’s Law, a product of power is produced when a voltage is dropped across a resistor and a current passed through the resistor. A Power Triangle superimposes the 3 quantities of power, voltage and current into a triangle since power is always consumed if a resistor is subjected to a voltage or if it conducts a current. The image below shows the power dissipated as heat in a resistor at the top and the current and the voltage at the bottom.

Figure 3 - Power triangle
Figure 3 - Power triangle

The above expression for the resistor power can produce two possible alternative variations if two of the values are known. The 3 standard formulas can be used to calculate the power dissipation of any resistor.

Formula - power dissipation of any resistor


V – is the voltage across the resistor in volts
– is the current flowing through the resistor in amperes
– is the resistance of the resistor in Ohms

Originally published at EEWeb

Premium Membership

Get access to premium HV/MV/LV technical articles, electrical engineering guides, research studies and much more! It helps you to shape up your technical skills in your everyday life as an electrical engineer.
More Information

Elaina Radon

EEWeb is a premier electrical engineering community that strives to offer its members the best online resources for hardware designers.

One Comment

  1. jaito
    Feb 04, 2013

    The formulation are incorrect.
    P= (i^2)*R=(v^2)/R

Leave a Comment

Tell us what you're thinking. We care about your opinion! Please keep in mind that comments are moderated and rel="nofollow" is in use. So, please do not use a spammy keyword or a domain as your name, or it will be deleted. Let's have a professional and meaningful conversation instead. Thanks for dropping by!

one  ×    =  7

Learn How to Design Power Systems

Learn to design LV/MV/HV power systems through professional video courses. Lifetime access. Enjoy learning!

Subscribe to Weekly Newsletter

Subscribe to our Weekly Digest newsletter and receive free updates on new technical articles, video courses and guides (PDF).
EEP Academy Courses - A hand crafted cutting-edge electrical engineering knowledge