Wattage Calculator
Calculate electrical power in watts and kilowatts
Circuit voltage (120V or 240V)
Current draw of the load
0 to 1 (1.0 for resistive loads)
Kilowatts
—
kW
Amps at 240V
—
amps
Watts
—
watts
Disclaimer: Results are estimates based on standard formulas. Not a substitute for professional advice from a licensed contractor, engineer, or architect. Verify all calculations before making purchasing or construction decisions.
How to Use This Calculator
Accurate wattage estimates help you size circuits, generators, and solar systems correctly. Enter your voltage, current, and power factor for instant results.
- Enter the circuit voltage (typically 120V or 240V)
- Enter the current draw in amps
- Set the power factor (1.0 for heaters, 0.85-0.9 for motors)
- Review watts, kilowatts, and equivalent amps at 240V
Expert Insights
Smart analysis for your Electrical project
Power Factor Awareness
Inductive loads like motors and transformers have power factors below 1.0, meaning they draw more current than real power used. AI flags low power factor scenarios.
Continuous Load derating
NEC requires continuous loads (3+ hours) be calculated at 125%. AI reminders keep your circuit sizing code-compliant for lighting and EV chargers.
240V Conversion Intelligence
The same wattage draws half the current at 240V versus 120V. AI shows both so you can choose the most efficient voltage for large loads.
Calculation Formula
Watts = Voltage x Current x Power Factor
Kilowatts = Watts / 1000
Amps at 240V = Watts / 240
Load Type Comparison
| Load Type | Power Factor | Typical Voltage | Example | Notes |
|---|---|---|---|---|
| Resistive | 1.0 | 120V/240V | Heater, incandescent | Watts = VA |
| Inductive (Motor) | 0.8-0.9 | 240V | AC, pump | Needs starting current |
| Electronic (SMPS) | 0.6-0.9 | 120V | Computer, LED driver | PFC varies |
Authority References
Our calculations are based on guidelines and standards from these authoritative sources:
- National Fire Protection Association (NFPA) — NFPA 70 — National Electrical Code (NEC). Learn more
- Institute of Electrical and Electronics Engineers (IEEE) — Standard 141 — Power Factor Correction. Learn more
- Underwriters Laboratories (UL) — Appliance Power Consumption Standards. Learn more
Frequently Asked Questions
How do you calculate watts from volts and amps?
For DC and resistive AC loads, watts = volts x amps. For AC loads with a power factor, multiply by the power factor: watts = volts x amps x power factor.
What is a power factor?
Power factor is the ratio of real power used by a load to apparent power delivered, ranging from 0 to 1. Resistive loads like heaters are 1.0, while motors may be 0.8-0.9.
How many amps is 1500 watts at 120V?
1500 watts divided by 120 volts equals 12.5 amps, which is why most space heaters use a dedicated 15-amp circuit.