# Resistor Wattage Calculator

Created by Mehjabin Abdurrazaque and Wojciech Sas, PhD candidate
Reviewed by Rijk de Wet
Last updated: Mar 03, 2022

The Omni resistor wattage calculator lets you figure out how much electrical power a resistor absorbs and dissipates as heat or light. This article also explains:

• How to determine the wattage of a resistor;
• The derivation of the electrical power formula for a resistor; and
• How to find the power dissipated by a resistor.

Our tool comes in handy in several ways. You can determine the unknown variables among resistance, power, voltage, and current from any two of these variables!

Alternatively, you can use this resistor power calculator to find the power dissipated by each resistor in a parallel circuit or a series circuit comprised of up to ten resistors! This part of our tool also functions as a parallel/series resistance calculator, voltage divider calculator, and current divider calculator. So, why not try it out?

## How to determine the wattage of a resistor?

We can't determine a resistor's wattage from its color code, but its size can help us here. A resistor's size varies according to its wattage or power rating. For example, the smallest carbon composition resistor's power rating is 1/8 W, while the largest resistor comes with a 5 W rating. A thick film chip resistor of size 20 x 10 mm has a power rating of 1/20 W, whereas a 250 x 120 mm sized thick film chip resistor's power rating is 1 W.

## What is the formula for electrical power?

We know that electricity is the flow of electrons. The potential difference $V$ is the amount of work done per unit charge to move a test charge from point A to B without changing its kinetic energy. The total work done when electrons flow through a resistor is:

$W = Q \cdot V$

where:

• $W$ — the total work done;
• $Q$ — the total charge of the electrons that passed through the resistor over a given period of time; and
• $V$ — the potential difference (or the potential drop) across the resistor.

We know that the current $I$ is the total charge flowing over a time period $\Delta t$:

$I = Q\ /\ \Delta t$

Thus, we can rewrite the work done as:

$W = \left(I \cdot \Delta t\right) \cdot V$

Power is the rate of work done, so the electrical power is:

$\begin{split} P &= W/ \Delta t \\ &= V\cdot I \end{split}$

⚠️ Do not confuse unit charge with an electron. A unit charge is one coulomb. The charge of an electron has a magnitude of 1.60217662 × 10-19 coulombs.

## How to find the power dissipated by a resistor?

To get the power dissipated by a resistor, we can begin with Ohm's law:

$V=I\cdot R$

where $R$ is the resistance of the resistor.

Therefore, we can rewrite the electrical power formula, $P = V \cdot I$, to estimate the power dissipated by the resistor as:

$\begin{split} P &= I^2 \cdot R \\ &= V^2 / R \\ \end{split}$

So we know what the formula for electrical power is, and we've learned all the theory about calculating the power dissipated by a resistor. Let's try to use this knowledge in practice!

## How to use the resistor wattage calculator?

Let's assume we have the following problem:

Three resistors of 20 Ω, 30 Ω, and 50 Ω are connected in series across a 125 V battery. Determine the total power dissipated by the resistors.

Let's see how to use the resistor wattage calculator to solve this problem:

1. Select the appropriate units for each quantity. The units of resistance, current, voltage, and power are ohm (Ω), ampere (A), volt (V), and watt (W) respectively by default.
2. Identify the variables given in the question — in the above question, the quantities given are resistance and voltage.
3. Enter 100 Ω (equivalent resistance) in the input box for resistance.
4. Enter 125 V in the input box for voltage.

There you have it! Our resistor power calculator displays both the current flown through (1.25 A) and the power dissipated (156.25 W) by the resistor.

## How to use the resistor wattage calculator for circuits with multiple resistors

To use the resistor wattage calculator for circuits with multiple resistors:

1. Select the circuit type from the drop-down list labelled Circuit type.
2. Choose the known parameter between the power supply's current and voltage from the drop-down list for My power supply has constant. Enter the known parameter's value in the next row.
3. Start entering the resistance of resistors from Resistor 1 (R₁). Each time you enter the value for resistance, a new row shows up to add the next resistance. You can add up to ten resistors.

Easy peasy! Our resistor power calculator displays the equivalent resistance, the current through each resistor, the voltage drop across each resistor, and the power dissipated in each resistor!

## Is this resistor wattage calculator suitable for resistors in AC circuits?

Our calculator uses the equation for power in a DC circuit to determine the power absorbed by a resistor, as given by $P = V\cdot I$. The average power of an AC circuit is the product of the root mean square (RMS) values of the voltage across and the current from the power supply, and the power factor:

$P = V_{RMS} \cdot I_{RMS} \cdot \text{PF}$

Here, $V_{RMS}$ and $I_{RMS}$ denote the RMS values of voltage and current. $\text{PF}$ is the circuit's power factor.

The RMS values of voltage and current are equivalent to a DC voltage and current respectively. For a purely resistive circuit (a circuit that contains only resistors and does not contain capacitors or inductors, or one where only resistors dissipate all circuit power), the power factor will be 1.

Hence, the power dissipated by a resistor in an AC circuit with no capacitors and inductors is $P = V_{RMS} \cdot I_{RMS}$. This means you can use our tool to calculate the power dissipated by a resistor in an AC circuit, but only if it's a purely resistive one.

## FAQ

### How does a resistor affect an electrical circuit?

Resistors slow down the electrons flowing in its circuit and reduce the overall current in its circuit. The high electron affinity of resistors' atoms causes the electrons in the resistor to slow down. These electrons exert a repulsive force on the electrons moving away from the battery's negative terminal, slowing them. The electrons between the resistor and positive terminal do not experience the repulsive force greatly from the electrons near the negative terminal and in the resistor, and therefore do not accelerate.

### Can a resistor supply power?

No. The process of supplying power involves converting other forms of energy into electrical energy. Resistors convert electrical energy into heat. So, a resistor cannot supply power to a circuit, but instead absorbs and dissipates power.

### How do I find the power dissipated by a 10 Ω resistor connected parallel to a 5 Ω resistor of 40 W?

In a parallel connection of resistors, the voltage across each resistor is the same.

1. Find the voltage (V) across resistor R1 of power rating P1 using the formula:
• V = √(P1 × R1).
2. Calculate the power dissipated by the second resistor (R2), P2 = V2/R2.
3. The overall voltage is 14.14 V, so the resulting power equals 20 W.

### How do I find which resistor dissipates the most power in a circuit?

The component with the greatest resistance dissipates the most power in a series circuit. In a series circuit, the same amount of current flows through all resistors, and power is the product of the square of the current and resistance, I2R.
In a parallel circuit, the component with the least resistance dissipates the most power in a parallel circuit, as the voltage across resistors remains the same, and power is the product of voltage and current (V×I).

### What are power resistors used for?

Power resistors are used for dissipating large amounts of energy as heat, as their resistance doesn't change significantly with rising temperatures.

Mehjabin Abdurrazaque and Wojciech Sas, PhD candidate
Single resisor circuit
Resistance (R)
Ω
Current (I)
A
Voltage (V)
V
Power (P)
W
Multiple resistors circuit
Circuit type
Parallel
My power supply has a constant
current
... of
A
Resistor 1 (R₁)
Ω
Resistor 2 (R₂)
Ω
You can add up to ten resistors — their fields will appear as you need them.
Input at least one resistor to obtain a result.
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