Calculate electrical power, voltage and current for DC, single-phase AC and three-phase AC systems. Then estimate appliance energy use and electricity cost.
Watt Calculator
LiveP = V × IHow to use the watt calculator
- Select the electrical system. Choose DC for batteries and many low-voltage circuits, single-phase AC for common household supplies, or three-phase AC for balanced commercial and industrial loads.
- Choose what to calculate. The calculator solves for watts, amps or volts and hides the unknown field.
- Enter the known values. Use a voltage preset or type the exact nameplate or measured value.
- Add power factor for AC. Use the equipment specification when available. A purely resistive load has a power factor close to 1.
- Review the breakdown. AC results also show apparent power in VA and reactive power in VAR.
If you know resistance instead of current, begin with the Ohm's Law Calculator. To convert the result into horsepower or another unit, use the Power Converter.
Energy use and electricity cost
Use the result above or enter the power printed on the appliance label.
Electricity Cost Calculator
Watt calculator formulas
| System | Formula |
|---|---|
| DC | P = V × I |
| Single-phase AC | P = V × I × PF |
| Three-phase AC | P = √3 × V × I × PF |
| Energy | kWh = (W ÷ 1,000) × hours |
| Cost | Cost = kWh × rate |
In these formulas, P is real power in watts, V is voltage in volts, I is current in amperes, and PF is power factor. For a deeper AC breakdown, the Power Factor Calculator connects watts, VA, VAR and phase angle.
Worked example
A 120-volt DC device drawing 10 amps uses 1,200 watts, or 1.2 kilowatts. Running it for four hours consumes 4.8 kWh. At $0.16 per kWh, it costs about $0.77 per day.
What the results mean
Watts (W) describe real power: the rate at which useful electrical energy is converted into heat, light, motion or another output. Volt-amperes (VA) describe apparent power in an AC circuit. Reactive volt-amperes (VAR) describe power that moves back and forth between the supply and reactive components. At a power factor of 1, watts and VA are equal.
Typical power-factor ranges
| Load type | Typical range | Examples |
|---|---|---|
| Resistive | About 1.00 | Incandescent lamps, simple resistance heaters |
| Small motors | About 0.60-0.85 | Fans, pumps, compressors |
| Corrected industrial loads | About 0.85-0.98 | Motor systems with correction equipment |
| Modern electronics | Varies widely | Computers, LED drivers, power supplies |
These ranges are general examples, not substitutes for the value printed on the equipment or measured with an appropriate instrument.
Watts, kilowatts and kilowatt-hours
Watts and kilowatts measure power; kilowatt-hours measure energy. A 2 kW heater running for three hours uses 6 kWh. This distinction matters because appliance labels usually show watts, while electricity bills charge for kWh.
Which value should you calculate?
| Known values | Calculate | Typical use |
|---|---|---|
| Volts and amps | Watts | Estimate device or circuit power |
| Watts and volts | Amps | Estimate current for planning |
| Watts and amps | Volts | Find the implied supply voltage |
| Watts and running time | kWh | Estimate energy and operating cost |
Electrical power versus mechanical power
This page calculates electrical wattage from volts, amps and power factor. It is intentionally different from the Physics Power Calculator, which calculates mechanical power from work and time. Separating the two topics keeps the formulas, assumptions and search intent clear.
From watts to a practical system estimate
Wattage is only one part of electrical planning. Long conductor runs can lose voltage, so use the Voltage Drop Calculator when distance matters. Conductor selection also depends on material, temperature and installation conditions; the Wire Gauge Calculator provides an estimate, but final selection must follow local codes and qualified professional guidance.
For renewable-energy planning, convert expected daily use into kWh and continue with the Solar Panel Size Calculator. The Energy Converter helps when source values are provided in joules, watt-hours or other energy units.
Accuracy and methodology
The calculator keeps full precision during intermediate steps and rounds displayed answers only for readability. Real equipment can have startup current, changing load, harmonic distortion, efficiency losses or a power factor that varies during operation. For details about how Numeros develops and reviews calculators, read the calculation methodology.
Assumptions that affect the result
Frequently asked questions
How many watts are in a kilowatt?
One kilowatt equals 1,000 watts. Divide watts by 1,000 to convert to kilowatts.
How do I calculate watts from volts and amps?
For DC, multiply volts by amps. For AC, include power factor. Three-phase AC also includes the square root of three.
Are watts and watt-hours the same?
No. Watts measure power at a moment in time. Watt-hours and kilowatt-hours measure energy over time.
Can this result select a wire or circuit breaker?
No. Sizing depends on electrical codes, installation method, temperature, distance and continuous-load rules. Use a qualified electrician.
Is anything I enter uploaded?
No. Calculations run locally in your browser.
Related calculators
Continue with the Engineering Calculators collection, or use a calculator matched to the next part of your calculation:
- Ohm's Law Calculator — solve voltage, current, resistance and power.
- Power Factor Calculator — compare real, apparent and reactive power.
- Voltage Drop Calculator — estimate voltage loss along a conductor.
- Wire Gauge Calculator — estimate conductor size using circuit inputs.
- Power Converter — convert watts, kilowatts, horsepower and other power units.
- Energy Converter — convert joules, watt-hours and energy units.
- Solar Panel Size Calculator — estimate array capacity from energy demand.
- Physics Power Calculator — calculate mechanical power from work and time.