Electrical Power Calculator - Watts, Volts, Amps and Ohms

Agarapu Ramesh — Editor and content reviewer
Editorially reviewed Reviewed by Agarapu Ramesh, science educator (chemistry). LinkedIn
Last reviewed: May 2026 | Standard electrical engineering formulas

Electrical power is the rate of doing work or making heat. In DC, P = V x I. In AC, power factor decides how much apparent power becomes real watts. That little PF box is not decoration.

Formula at a glance

  • DC power: W = V x A
  • resistive power: W = V^2 / R = A^2 x R
  • AC real power: W = V x A x PF

Field note: A clamp meter that only shows amps is not a power meter. For mixed AC loads, you need voltage and PF too.

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Power Calculator

Calculate electrical power (watts)

Enter any two values to calculate the others:

V
A
Ω
W
Result
Enter 2 values

Power Formulas

BasicP = V × I
With RP = I²R = V²/R

Units

1 kW = 1000 W
1 MW = 1,000,000 W
1 HP ≈ 746 W
1 kWh = 1 kW for 1 hour

Related

Ohm's LawWatts to AmpskW to Amps

How to use the Electrical Power Calculator

Use this as a fast electrical check, then compare the result with the nameplate, measured voltage and power factor. The formula is clean. Real panels, motors and UPS loads usually have one extra wrinkle.

Worked example

Example: 230 V at 10 A is 2,300 VA. At PF 0.8, real power is 1,840 W.

Practical checks before you trust the number

  • Use watts for heat, work and energy bills.
  • Use VA for transformer, UPS and inverter current capacity.
  • For three-phase real power, use 1.732 x line voltage x line current x PF.

Common mistake

A clamp meter that only shows amps is not a power meter. For mixed AC loads, you need voltage and PF too.

Sources and references

Related calculators

Frequently Asked Questions

P = V × I for DC. For single-phase AC, P = V × I × PF. For three-phase, P = √3 × V × I × PF. Example: 230 V × 5 A × 0.9 = 1035 W. Always pick the right formula for your circuit type — DC, single-phase, or three-phase — or your power figure will be off.

DC: P = V × I. Or P = I² × R, or P = V² ÷ R using Ohm's law. Example: 24 V × 5 A = 120 W; or 5² × 4.8 = 120 W; or 24² ÷ 4.8 = 120 W. All three give the same answer for any DC resistive circuit.

Single-phase AC: P = V × I × PF. Example: 230 V × 10 A × PF 0.85 = 1955 W. The PF accounts for the phase angle between voltage and current. For purely resistive loads (heaters, incandescent), PF = 1 and the formula simplifies to V × I.

Three-phase: P = √3 × V_line × I_line × PF. Example: 415 V × 20 A × PF 0.85 = 12.2 kW. Always use line voltage and line current. The √3 factor appears because three-phase power is balanced across three line currents at 120° phase offset.

PF (power factor) is the cosine of the phase angle between voltage and current. P = V × I × PF, so lower PF means less real power for the same V × I (apparent power). At PF 0.5, half your apparent power is reactive — flowing back and forth doing no work. PF correction with capacitors fixes this.

P = V × I = 12 × 5 = 60 W (DC). For AC at PF 0.9: 12 × 5 × 0.9 = 54 W. So a 12 V, 5 A load is 60 W on DC and somewhat less on AC depending on PF. Most low-voltage residential applications (LED strips, doorbells) are DC, so the simple V × I rule applies.

Yes, that's the main use. Sum your equipment loads in watts to get total connected load. Apply diversity (typically 0.7 to 0.9 for residential) to estimate actual demand. Compare against feeder rating and breaker setting. Standard practice for any panel sizing or service entry calculation.