Calculate electrical power three ways, then estimate its electricity running cost.
How the Electrical Power Calculator Works
Electrical power (measured in watts) can be calculated three equivalent ways depending on which two quantities you already know, all derived from Ohm's Law (V = I × R). Pick the formula that matches your known values: Voltage × Current, Current² × Resistance, or Voltage² ÷ Resistance — all three give the identical result for a consistent circuit, which is a useful way to cross-check your numbers.
P = V × I (Power = Voltage × Current)
P = I² × R (Power = Current² × Resistance)
P = V² ÷ R (Power = Voltage² ÷ Resistance)
From Watts to Running Cost
Once you know a device or circuit's power draw in watts, you can estimate what it costs to run. First convert watts to kilowatt-hours (kWh) — the unit utility companies bill by — using Daily kWh = Power(W) ÷ 1,000 × Hours Used, then multiply by your electricity rate to get a daily cost, and by 30 for a rough monthly figure.
When to Use Each Formula
Use P = V × I when you know a device's rated voltage and the current it draws (common on appliance nameplates). Use P = I² × R when you know the current through a resistive load and its resistance (common in circuit analysis). Use P = V² ÷ R when you know the voltage across a resistive load and its resistance — useful for calculating power dissipated by a specific resistor in a circuit.
❓ Frequently Asked Questions
Why do all three power formulas give the same answer? ▼
Because they're all derived from Ohm's Law (V = I × R). Substituting I = V/R into P = VI gives P = V²/R, and substituting V = IR into P = VI gives P = I²R — they're mathematically equivalent expressions of the same relationship, just rearranged for whichever two values you know.
What's the difference between watts and kilowatt-hours? ▼
Watts (W) measure instantaneous power — how fast energy is being used at a given moment. Kilowatt-hours (kWh) measure total energy consumed over time (power × hours ÷ 1,000), which is the unit your electricity bill is actually calculated in.
How do I find the voltage, current, or resistance of my device? ▼
Check the device's nameplate or spec sheet, which often lists voltage and either current (amps) or wattage directly. For resistive components like heating elements, a multimeter can measure resistance directly, and current can often be measured with a clamp meter.
Does this calculator work for AC circuits with power factor? ▼
This calculator uses the basic DC/resistive power formulas (P = VI, I²R, V²/R), which apply directly to DC circuits and to AC circuits with a purely resistive load and power factor of 1. For AC circuits with reactive loads (motors, some electronics), real power also depends on the power factor, which isn't included here.
How accurate is the running cost estimate? ▼
It's a straightforward estimate assuming constant power draw for the hours you enter and a flat electricity rate. Real-world costs can vary with time-of-use utility rates, taxes and fees on your bill, and devices that don't draw constant power throughout their use.
🏆 About This Tool — Accuracy & Trust
🔒 Data Privacy: All calculations run entirely within your browser using JavaScript. Nothing you enter here is ever transmitted to our servers, stored, sold, or shared.
📐 Accuracy Note: This tool uses standard, widely published formulas and guidelines. Results are estimates for informational purposes; for financial, medical, or engineering decisions, consult a licensed professional.
📅 Last Updated: September 2026.