Resistor Wattage Calculator

Created by Luciano Mino
Last updated: Aug 31, 2022

Our resistor wattage calculator can obtain the wattage of any resistor or a combination of resistors within a circuit (in series or parallel).

With our tool, you can use any combination of resistance, current, and voltage to find the power dissipated by a resistor.

If you don't know what any of these mean, don't worry. The short text below explains everything you need to know about resistors and the resistor power formula.

Keep reading to learn more!

What is the wattage of a resistor?

We use the word wattage when we describe power in terms of watts.

When current flows through a resistor in a circuit, it dissipates electrical power in the form of heat.

A resistor's wattage is the maximum amount of power a resistor can absorb or dissipate without sustaining damage.

Let's look at the resistor power formula to know how to calculate its wattage.

See how to convert watts to heat in our watts to heat calculator.

Resistor power formula

To find the electrical power absorbed by a resistor, we need to find the amount of work required to move electrons through it:

W=QVW = QV

where:

  • WW is the work in joules.
  • QQ is the amount of charge transferred.
  • VV is the voltage difference across the resistor. Voltage represents the amount of work per unit charge required to move a charge between two points.

From its definition, the electric current is the amount of charge per unit of time that passes through a component in a circuit:

I=QΔtI = \frac{Q}{\Delta t}

And we can replace this expression in the previous formula to get the resistor power formula:

P=WΔt=IVP = \frac{W}{\Delta t} =IV

Another way to write this expression is in terms of the resistor's resistance RR. From Coulomb's law, it's trivial to obtain:

P=I2RP=V2R\begin {align*} P &= I^{2} R \\ P &= \frac{V^{2}}{R} \end {align*}

🙋 With our resistor wattage calculator, you can input any two of these parameters, and it will automatically obtain the resistor's wattage for you!

Resistor wattage in series and parallel

We've shown the simplest scenario where we calculate the power dissipated by only one resistor.

But what if we had two or more connected in series? What happens if, instead, we connect them in parallel?

When we connect resistors in series inside a circuit with a voltage source V, the current flowing through them is the same. Hence, we can use this to calculate the power dissipated by each resistor.

The total power dissipated is the sum of all these resistor wattages:

Ptot=I2R1+I2R2+....+I2Rn{\small P_{tot} = I^{2} R_{1} + I^{2} R_{2} + .... + I^{2} R_{n}} \\
Ptot=I2ReqP_{tot} = I^{2} R_{eq}

where:

  • PtotP_{tot} is the total power dissipated by all the resistors according to the resistor power formula (or wattage of the resistor);
  • II is the current; and
  • ReqR_{eq} is the equivalent resistance.

On the other hand, if we connect them in parallel, the voltage difference across each resistor is the same.

In this case, we can use:

Ptot=V2R1+V2R2+....+V2Rn{\small P_{tot} = \frac{V^{2}}{R_{1}} + \frac{V^{2}}{R_{2}} + .... + \frac{V^{2}}{R_{n}}}\\
Ptot=V2ReqP_{tot} = \frac{V^{2}}{R_{eq}}

where:

  • VV is the power source's voltage; and
  • ReqR_{eq} is the equivalent resistance for resistors in parallel.

✅ Or try our resistor wattage calculator! With it, you can easily find the resistor wattage in series or parallel (up to ten resistors).

Luciano Mino
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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