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Watts Calculator

Calculating electrical wattage answers the practical question: "How much power does this appliance draw, and how much will it cost to run per hour?" I

Electrical
A
Current draw in amps
V
Supply voltage
Single-phase or three-phase line
1.0 for resistive heating, 0.8–0.9 for motors/electronics
$/kWh
Electricity tariff rate for running cost estimation
Formula & Methodology
Single-Phase: P = V × I × PF; Three-Phase: P = √3 × V × I × PF; Running Cost/hr = (Watts ÷ 1000) × Electricity Rate

Watts are computed by multiplying voltage by current and the power factor. When the power factor is 1.0 (pure resistive loads), Watts equal Volt-Amps. For devices with motors or power supplies (where PF < 1.0), real power in Watts is strictly less than Volt-Amps. Dividing Watts by 1,000 gives kilowatts, which multiplied by your utility rate yields the running cost per hour.

Calculated Real Power (Watts)

1,080 W 1.08 kW

Overview

Calculating electrical wattage answers the practical question: "How much power does this appliance draw, and how much will it cost to run per hour?" In electrical terms, real power in watts represents the true rate of energy consumed to perform work, while apparent power in volt-amperes represents the total power flowing through the circuit.

Safety Notice: Wattage and running cost calculations are planning estimates. Actual device power consumption fluctuates with compressor cycling, variable speed motor drives, and temperature. Consult appliance nameplate ratings and hire a certified electrician for circuit verification.

Worked Examples

120V Circuit Drawing 10A with 0.9 Power Factor

Voltage = 120V, Current = 10A, PF = 0.9, Electricity Rate = $0.16/kWh.

  1. Calculate Apparent Power: S = 120 V × 10 A = 1,200 VA
  2. Calculate Real Power: P = 1,200 VA × 0.9 = 1,080 W (1.08 kW)
  3. Calculate Hourly Running Cost: 1.08 kW × $0.16 = $0.1728 / hour
Final Answer: 1,080.0 W (1.08 kW) | Apparent: 1,200 VA | Cost: $0.1728 / hour

240V Air Conditioner Drawing 15A with 0.85 PF

Voltage = 240V, Current = 15A, PF = 0.85, Electricity Rate = $0.15/kWh.

  1. Apparent Power: 240 × 15 = 3,600 VA
  2. Real Power: 3,600 × 0.85 = 3,060 W (3.06 kW)
  3. Hourly Cost: 3.06 kW × $0.15 = $0.459 / hour
Final Answer: 3,060.0 W (3.06 kW) | Cost: $0.459 / hour

Simple Resistive 120V 12.5A Space Heater (PF = 1.0)

Voltage = 120V, Current = 12.5A, PF = 1.0, Rate = $0.20/kWh.

  1. Real Power: 120 V × 12.5 A × 1.0 = 1,500 W (1.5 kW)
  2. Hourly Cost: 1.5 kW × $0.20 = $0.30 / hour
Final Answer: 1,500.0 W (1.50 kW) | Cost: $0.30 / hour

How to Use This Calculator

  1. Enter the current draw in amperes and the circuit voltage.
  2. Select single-phase or three-phase power.
  3. Enter the load power factor (use 1.0 for heating elements, 0.85–0.9 for motors).
  4. Enter your electricity tariff rate in $/kWh to view hourly and daily operating costs.
  5. Click Calculate to see real power in watts and kilowatts alongside hourly operating costs.

Reference Table

Device TypeTypical Amps at 120VPower Factor (PF)Real Power (Watts)Est. Cost per 8h (@$0.16)
LED Television0.8 A0.9086 W$0.11
Desktop Computer + Monitor2.5 A0.85255 W$0.33
Portable Space Heater12.5 A1.001,500 W$1.92
Dehumidifier4.2 A0.80403 W$0.52
Window Air Conditioner8.5 A0.85867 W$1.11

Frequently Asked Questions

How do I convert amps and volts to watts?
For standard DC or single-phase resistive circuits, multiply amps by volts (Watts = Volts × Amps). For AC motors or electronic devices, multiply Volts × Amps × Power Factor.
Why are Watts and Volt-Amps different on my calculator?
Volt-Amps (VA) measures total electrical power flowing through the wires. Watts (W) measures the portion of that power actually consumed to generate mechanical work, light, or heat. The ratio between them is the power factor.
How many watts can a standard 15-amp household outlet handle?
A standard 120V 15A outlet can support a theoretical maximum of 1,800 watts (120 × 15). Under the 80% continuous load safety guideline, continuous appliances should not exceed 1,440 watts.
What is a kilowatt (kW) compared to a watt (W)?
One kilowatt equals exactly 1,000 watts. For example, a 1,500-watt hair dryer draws 1.5 kW of electrical power.
Does running a 1,500-watt appliance for 1 hour consume 1.5 kWh?
Yes. Energy consumed in kilowatt-hours equals power in kilowatts multiplied by time in hours: 1.5 kW × 1 hour = 1.5 kWh.

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Reviewed & Authored by: OnlineConvers Engineering Editorial Team · Last Updated: 2026-10-07
Watts calculations apply the fundamental real power definition P = V × I × PF for single-phase circuits and P = √3 × V × I × PF for three-phase circuits. Running cost calculations multiply active kilowatts (W / 1000) by user-entered utility rates. Voltage, current, and power factor inputs are guarded against negative values and out-of-range power factor entries.
Citations: Electrical Power Measurements and Wattage Calculations (IEEE Transactions on Power Delivery) · US Energy Information Administration (EIA) Electric Power Monthly (US Department of Energy)