1 watt is 1 joule per second. In circuit terms, 1 W = 1 V·A.

Why are there multiple power formulas?

Because combining P = V·I with Ohm’s Law (V = I·R) gives P = I²R and P = V²/R.

What if I enter mA or kΩ?

That’s fine—this calculator converts all inputs to base units internally.

Electrical Power Calculator

Q: Can I use this for AC circuits?

For a purely resistive AC load, treat RMS values as DC and use P = V times I. When voltage and current are out of phase, use the P = V times I times PF method, which accounts for the gap between real and apparent power.

Q: What is power factor?

Power factor (PF, equal to cos of the phase angle) is the ratio of real power delivered to apparent power supplied, PF = P/S where S = V times I. It is entered as a decimal between 0 and 1.

Solve for power (W), voltage (V), current (A), or resistance (Ω) using P = V·I, P = I²R, P = V²/R, Ohm’s Law: V = I·R, or (for AC circuits) P = V·I·PF. Includes unit conversion, quick picks, step-by-step, and a mini power scale.

Background

Electrical power tells you how fast electrical energy is used or delivered. For simple DC circuits, power links voltage and current: P = V·I. Using Ohm’s Law V = I·R, you can also write P = I²R and P = V²/R. Those three formulas assume voltage and current stay in phase (a purely resistive load) — for AC circuits where they don't, use P = V·I·PF below, which accounts for the gap between real and apparent power using the power factor (PF = cos φ).

Choose a method

Method:

P = V · I (Voltage × Current)

P = I² · R (Current² × Resistance)

P = V² / R (Voltage² ÷ Resistance)

V = I · R (Ohm’s Law — solve V, I, or R)

P = V · I · PF (AC Power with Power Factor)

Tip: Use Ohm’s Law when you want current, voltage, or resistance directly from the other two. Use the P-forms when you’re solving power (or solving V/I/R from power). Use P = V·I·PF for AC circuits where voltage and current are out of phase.

Solve for

We’ll disable options that aren’t solvable with your chosen method.

Power (P)

Voltage (V)

Current (I)

Resistance (R)

Resistance must be > 0 when used in division or square roots.

Power Factor (PF = cos φ)

Enter as a decimal between 0 and 1. This simplified model treats PF as a magnitude only — it doesn't distinguish leading vs. lagging or compute reactive power.

Quick picks:

Chips prefill common electrical scenarios and run the calculation.

Options:

Show step-by-step

Show mini power scale

Result:

No results yet. Enter values and click Calculate.

How to use this calculator

Pick a method: P = V·I, P = I²R, P = V²/R, V = I·R, or P = V·I·PF.
Choose what you want to solve for (P, V, I, R, or PF).
Enter the other required values (units can differ; we convert internally).
Click Calculate to see the result, conversions, and steps.

How this calculator works

Convert inputs to consistent base units: W, V, A, Ω.
Solve for the selected variable using the chosen equation.
Display common unit equivalents (e.g., W ↔ kW ↔ hp, Ω ↔ kΩ).
For the AC power-factor method, also compute the apparent power S = V·I (in VA) alongside the real power, so you can see the gap PF accounts for.
Optionally show a mini power scale for intuition.

Formula & Equation Used

P = V·I

P = I²·R

P = V² / R

V = I·R

P = V·I·PF, where PF = P/S and S = V·I is the apparent power (VA)

Identity: 1 W = 1 V·A

Example Problems & Step-by-Step Solutions

Example 1 — P = V·I

A charger outputs 5 V at 2 A.

Use P = V·I
P = 5 × 2 = 10 W

Example 2 — P = V²/R

A device runs at 12 V with 6 Ω resistance.

Use P = V²/R
P = 12² / 6 = 144 / 6 = 24 W

Example 3 — P = I²R

A current of 0.5 A flows through a 10 Ω resistor.

Use P = I²R
P = (0.5)² × 10 = 0.25 × 10 = 2.5 W

Example 4 — P = V·I·PF

An AC motor draws 5 A at 120 V with a power factor of 0.9.

Apparent power: S = V·I = 120 × 5 = 600 VA
Real power: P = S × PF = 600 × 0.9 = 540 W

Frequently Asked Questions

Q: What is 1 watt?

1 watt means 1 joule per second. In circuit terms, 1 W = 1 V·A.

Q: Can I use this for AC circuits?

For a purely resistive AC load (voltage and current in phase), treat RMS values as you would DC and use P = V·I. When voltage and current are out of phase, use the P = V·I·PF method instead — it accounts for the gap between real and apparent power, though it doesn't compute the phase angle or reactive power directly.

Q: What is power factor?

Power factor (PF = cos φ) is the ratio of real power actually delivered to the apparent power supplied (PF = P/S, where S = V·I). It's entered here as a decimal between 0 and 1 — this calculator doesn't distinguish leading from lagging power factor.

Q: Why are there multiple formulas for power?

Because combining P = V·I with V = I·R gives P = I²R and P = V²/R.

Q: What if I enter kΩ or mA?

That’s fine — the calculator converts everything to base units internally.