Calculate electrical power using voltage, current, or resistance
Electrical power — measured in watts (W) — is the rate at which a circuit converts electrical energy into another form (heat, light, motion). There are three equivalent formulas depending on which quantities you know:
| Known values | Formula | Example |
|---|---|---|
| Voltage + Current | P = V × I | 12V × 2A = 24W |
| Current + Resistance | P = I² × R | 2A² × 6Ω = 24W |
| Voltage + Resistance | P = V² ÷ R | 12V² ÷ 6Ω = 24W |
All three formulas are mathematically equivalent — they are all derived from Ohm's Law (V = I × R). Use whichever matches the values you have available.
Power (watts) tells you the rate of energy use. Multiply power by time to get energy consumed in watt-hours (Wh) or kilowatt-hours (kWh). Multiply kWh by your electricity rate to get cost.
A 60W light bulb running for 10 hours uses 60 × 10 = 600 Wh = 0.6 kWh. At $0.20/kWh, that costs $0.12. Run it every day for a year: 0.6 × 365 × $0.20 = $43.80 per year for one bulb.
| Device | Typical power |
|---|---|
| LED light bulb | 8–12 W |
| Smartphone charger | 5–20 W |
| Laptop | 30–80 W |
| Desktop computer | 150–400 W |
| Microwave | 800–1,200 W |
| Hair dryer | 1,200–2,000 W |
| Electric kettle | 1,500–3,000 W |
| Electric car charger (L2) | 3,600–11,000 W |
For simple resistive loads like heaters and incandescent bulbs, the power calculated with P = V × I is the real power — all of it is converted to useful work (heat or light). For devices with motors, transformers, or capacitors (fridges, AC units, fluorescent lights), the relationship is more complex. These devices draw reactive power in addition to real power, and the ratio between them is called the power factor. A power factor below 1 means the device draws more current than its real power suggests — relevant for wiring and inverter sizing but not for electricity billing (meters measure real power).
Multiply voltage by current. A circuit running at 120V with 5A of current consumes 120 × 5 = 600W of power.
1 kilowatt (kW) = 1,000 watts (W). Kilowatts are used for larger devices and systems. Your electricity bill is based on kilowatt-hours (kWh) — one kWh is 1kW running for one hour, or 1,000W running for one hour.
Power calculations are essential for selecting the right fuse or circuit breaker, choosing the correct wire size, sizing inverters and generators, calculating electricity costs, and understanding the energy demand of any electrical system.
Power factor is the ratio of real power (what does actual work) to apparent power (total power drawn from the supply). Resistive loads like heaters have a power factor of 1. Motors and fluorescent lights typically have power factors of 0.7–0.9, meaning they draw 10–30% more current than their real power alone would suggest.
A standard US outlet at 120V on a 15A circuit can supply up to 1,800W (120 × 15). For safety, continuous loads should not exceed 80% — 1,440W. European outlets at 230V on a 16A circuit can supply up to 3,680W, with 2,944W as the safe continuous limit.