Electrical Power Calculator | Watts, Volts, Amps

Quick Start Guide

  1. Select circuit type: Choose DC, AC single-phase, or AC three-phase based on your system.
  2. Choose known variables: Select voltage & current (VI), current & resistance (IR), or voltage & resistance (VR).
  3. Enter values: Input the known electrical parameters. For AC circuits, set power factor if needed.
  4. View results: See real power (watts), voltage, current, and resistance.
  5. Calculate energy cost (optional): Check "Calculate Energy Consumption & Cost" to estimate daily/yearly costs with operating hours and rate.

Understanding Power Calculations

Three equivalent ways to compute power

Power (P, in watts) can be found from any two of voltage, current, and resistance: P = V × I, P = I² × R, or P = V² ÷ R. Pick whichever variable set (V&I, I&R, or V&R) matches the values you already know.

DC vs. AC: where power factor comes in

DC power is simply P = V × I. AC power is reduced by the power factor (PF), which accounts for the phase difference between voltage and current in circuits with motors, transformers, or other reactive loads: single-phase AC uses P = V × I × PF, three-phase uses P = √3 × V × I × PF.

What this calculator covers — and what it does not

This calculator computes real power (watts) — the power actually converted to useful work or heat. It does not compute apparent power (kVA) or reactive power (kVAR), which describe the total current a supply must deliver in AC circuits with reactive loads.

From power to a monthly electricity bill

Power tells you the instantaneous rate of energy use; multiplying by how long the load actually runs gives energy (kWh = watts × hours ÷ 1000), and multiplying energy by your utility rate gives cost. Check "Calculate Energy Consumption & Cost" to do both steps from a daily usage estimate.

Features

DC & AC Support: Handle DC, single-phase AC, and three-phase AC power calculations.

Multiple Variable Sets: Calculate with voltage-current, current-resistance, or voltage-resistance pairs.

Power Factor Adjustment: Apply a configurable power factor to compute real power in AC circuits.

Energy Cost Calculation: Estimate electricity costs with operating hours and rate per kilowatt-hour.

Export: Export your calculation to PDF, JSON, or CSV.

Common Use Cases

Electrical Load Analysis: determine power consumption of appliances and equipment, calculate circuit breaker and wire gauge requirements, and verify power ratings match actual operating conditions.

Motor & Machine Sizing: size motors based on voltage and current requirements, calculate mechanical power output from electrical input, and factor in power factor for AC motor efficiency.

Energy Cost Estimation: calculate daily/monthly/yearly electricity consumption, estimate operating costs for continuous equipment, and compare energy usage across different operating scenarios.

System Design & Troubleshooting: verify power budgets for electrical installations, troubleshoot voltage/current imbalances, and validate electrical component ratings under load.

Frequently Asked Questions

DC (direct current) power is P = V × I. Single-phase AC uses RMS voltage and current with a power factor: P = V × I × PF. Three-phase power is P = √3 × V × I × PF and is used in industrial and commercial supply. Select your circuit type in this calculator for the correct formula.
For DC circuits: P = V × I (Voltage × Current). For single-phase AC: P = V × I × Power Factor. For 3-phase AC: P = √3 × V_line × I_line × PF. Select your circuit type in this calculator for the correct formula.
Energy (kWh) = Power (W) × Time (hours) / 1000. Cost = kWh × electricity rate. For example, a 1,500W heater running 8 hours at $0.12/kWh: 1.5 × 8 × $0.12 = $1.44/day. Check "Calculate Energy Consumption & Cost" in this calculator to project daily, monthly, and annual costs.
Power factor (PF) is the ratio of real power (W) to apparent power (VA). PF = 1.0 is ideal (purely resistive load). Motors, transformers, and fluorescent lights typically have PF of 0.7-0.95. A low PF means you draw more current for the same usable power, increasing losses and utility bills.
Yes. Choose the variable set that matches what you know — voltage & current, current & resistance, or voltage & resistance — and the calculator solves for power along with the other unknowns. For AC, set the power factor so the result matches real-world measurements.

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