Resistor Wattage Calculator

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?

We can't determine a resistor's wattage from its color code (learn how to calculate it with our resistor color code calculator!), 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 $1/8\ \mathrm{W}$, while the largest resistor comes with a $5\ \mathrm{W}$ rating. A thick film chip resistor 0201 of size $0.6 \times 0.3\ \mathrm{mm}$ has a power rating of $1/20\ \mathrm{W}$, whereas a 2512 ($6.3 \times 3.2\ \mathrm{mm}$ sized) thick film chip resistor's power rating is $1\ \mathrm{W}$.

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 across the resistor: you can calculate it with our voltage drop calculator.

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

Thus, we can rewrite the work done as:

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

To get the power dissipated by a resistor, we can begin by calculating 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!

Let's assume we have the following problem:

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 ($\mathrm{Ω}$), ampere ($\mathrm{A}$), volt ($\mathrm{V}$), and watt ($\mathrm{W}$) respectively by default.

2. Identify the variables given in the question — in the above question, the quantities given are resistance and voltage.

3. Enter $100\ \mathrm{Ω}$ (equivalent resistance) in the input box for resistance.

4. Enter $125\ \mathrm{V}$ in the input box for voltage.

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

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 ($R_1$). 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!

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, $V_{\mathrm{RMS}}$ and $I_{\mathrm{RMS}}$ 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: if you are confused, our RMSvoltage calculator is here to help! 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 $P = V_{\mathrm{RMS}} \cdot I_{\mathrm{RMS}}$. 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.