Electrical Watts Calculator
Enter any two values (Volts, Amps, Watts), leave one blank, and click calculate. Optional fields can estimate daily energy use and monthly electricity cost.
What Is a Watt?
A watt (W) is a unit of power. It tells you how fast electrical energy is being used. If a device uses 100 watts, it consumes energy faster than a 10-watt device.
In basic electrical calculations, power is connected to voltage and current using this formula: Watts = Volts × Amps. For many AC loads, power factor also matters: Watts = Volts × Amps × Power Factor.
Core Formulas Used in This Calculator
1) Solve for Power (Watts)
P = V × I × PF
- P = power in watts
- V = voltage in volts
- I = current in amps
- PF = power factor (default 1)
2) Solve for Current (Amps)
I = P / (V × PF)
3) Solve for Voltage (Volts)
V = P / (I × PF)
Energy and Cost Estimates
Power tells you the rate of energy use, while energy bills are based on kilowatt-hours (kWh). Once watts are known, the calculator also estimates:
- Daily energy: (Watts × Hours per day) ÷ 1000
- Monthly energy: Daily kWh × 30
- Monthly cost: Monthly kWh × electricity rate
Practical Examples
Example A: Laptop Charger
Suppose your charger runs at 20V and 3A with PF = 1. Power is 20 × 3 × 1 = 60W.
Example B: Small Heater
A heater draws 12.5A on a 120V line. Power is 120 × 12.5 = 1500W. If used 2 hours/day, energy is 3.0 kWh/day.
Example C: Estimating Bill Impact
A 100W device running 8 hours/day uses 0.8 kWh/day. At $0.15/kWh, that is about $0.12/day, or about $3.60/month.
Tips for Accurate Results
- Use measured values from a meter when possible.
- For AC motors and compressors, include a realistic power factor.
- Keep units consistent: volts, amps, watts, and kWh.
- Treat cost results as estimates; utility tariffs can vary by season and time-of-use.
Safety Reminder
This calculator is for planning and estimation. Electrical systems can be hazardous. If you are unsure about wiring, load limits, breaker sizing, or code compliance, consult a licensed electrician.