# Resistor Wattage Calculator

- How to determine the wattage of a resistor?
- What is the formula for electrical power?
- How to find the power dissipated by a resistor?
- How to use the resistor wattage calculator?
- How to use the resistor wattage calculator for circuits with multiple resistors
- Is this resistor wattage calculator suitable for resistors in AC circuits?
- FAQ

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:

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 $\Delta t$:

Thus, we can rewrite the work done as:

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

⚠️ 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**.

*To learn more about charges, visit Omni's electrostatic calculators on Coulomb's law and electric field!*

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

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

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:

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:

**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.**Identify**the variables given in the question — in the above question, the quantities given are resistance and voltage.**Enter**`100 Ω`

(equivalent resistance) in the input box for resistance.**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**:

**Select**the circuit type from the drop-down list labelled`Circuit type`

.**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.**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:

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**.

**Find**the voltage (V) across resistor R_{1}of power rating P_{1}using the formula:- V = √(P
_{1}× R_{1}).

- V = √(P
**Calculate**the power dissipated by the second resistor (R_{2}), P_{2}= V^{2}/R_{2}.- 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, **I ^{2}R**.

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.

*Want to learn more about resistors? Check out our wire resistance calculator!*

*Input at least one resistor to obtain a result.*