Electrical Engineering

Electrical Power Calculator

Calculate electrical power from any two of voltage, current and resistance.

Formula P = V × IReviewed Aug 14, 2026

Electrical power is the rate at which a circuit does work - it's calculated as voltage multiplied by current, P = VI. Combined with Ohm's Law (V = IR), that single relationship rearranges into two other useful forms: P = I²R and P = V²/R. Select which two quantities you know and this calculator returns the power.

Calculation Bench

Known Quantities

01Voltage

V · Electrical potential difference across the circuit.

02Current

I · Rate of electric charge flow through the circuit.

Solution

Enter the required values to calculate power.

P = V × I

Formula Sheet

P = V \times I

P = I^2 R

P = \dfrac{V^2}{R}

PPower
VVoltage
ICurrent
RResistance

Variables & Units

Symbol	Variable	Description	Common Units
P	Power	Rate of electrical energy transfer.	W, kW
V	Voltage	Electrical potential difference across the circuit.	V, mV, kV
I	Current	Rate of electric charge flow through the circuit.	A, mA
R	Resistance	Opposition to current flow presented by the load.	Ω, kΩ, MΩ

How to Use This Calculator

01Select which two quantities you know - Voltage & Current, Current & Resistance, or Voltage & Resistance.
02Enter the two known values, choosing the correct unit for each.
03Select Calculate to see the resulting power.
04Switch the selector to explore the other forms if you have a different pair of known values.

How the Formula Works

Power is the rate of energy transfer - for a simple resistive circuit, it equals voltage multiplied by current: P = VI. Combining that with Ohm's Law (V = IR) lets you substitute for either V or I, producing two equivalent forms: replacing V with IR gives P = I²R, and replacing I with V/R gives P = V²/R.

All three forms describe the exact same physical quantity - for any circuit that actually obeys Ohm's Law, all three give the identical power figure, so which one to use is purely a matter of which two quantities you happen to know.

Worked Example 01

Power from voltage and current

Known

Voltage (V): 120 V
Current (I): 2 A

Formula

P = V × I

Substitution

P = 120 × 2

Result

P = 240 W

A 120 V supply driving a 2 A current delivers 240 W of power to the load.

Worked Example 02

Power from current and resistance

Known

Current (I): 2 A
Resistance (R): 50 Ω

Formula

P = I² × R

Substitution

P = 2² × 50

Result

P = 200 W

A 2 A current through a 50 Ω resistor dissipates 200 W as heat.

Worked Example 03

Power from voltage and resistance

Known

Voltage (V): 120 V
Resistance (R): 60 Ω

Formula

P = V² / R

Substitution

P = 120² / 60

Result

P = 240 W

A 120 V supply across a 60 Ω load delivers 240 W - the same result as the first example, since these values also satisfy Ohm's Law for a 2 A current.

Applications

01Estimating power dissipation or consumption of a resistive load
02Sizing a power supply, fuse, or heatsink for a known load
03Checking a component's power rating against its expected operating conditions

Assumptions

01The load behaves as an ideal, linear (ohmic) resistor.
02The circuit is a steady-state DC circuit, or a purely resistive AC load.
03Power here means real (dissipated) power - reactive power in AC circuits with inductance or capacitance isn't modeled.

Where This Model Stops

01Does not apply to AC circuits with reactance (inductors, capacitors) - those require the power factor and apparent/reactive power, not this simple resistive relationship.
02Not valid for non-ohmic components such as diodes or transistors, whose resistance varies with voltage or current.

References

[1]
Electrical power
Standard electrical engineering and physics fundamentals
P = VI, with P = I²R and P = V²/R as the Ohm's-Law-substituted equivalent forms.

Frequently Asked Questions

Why are there three different power formulas?

They're all the same relationship, P = VI, algebraically rearranged using Ohm's Law (V = IR) to use whichever two quantities you happen to know - voltage and current, current and resistance, or voltage and resistance.

What's the difference between power and energy?

Power (watts) is the rate of energy transfer at an instant. Energy (typically kilowatt-hours) is power sustained over time - multiply power by the duration it's applied to get energy consumed.

Does this work for AC circuits?

Only for a purely resistive AC load, where voltage and current stay in phase. Circuits with inductance or capacitance shift current out of phase with voltage, requiring power factor and separate real/reactive/apparent power calculations that this tool doesn't cover.

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