Wattage Calculator
Use this tool to calculate watts, amps, or volts for DC, single-phase AC, and three-phase AC systems.
What is wattage?
Wattage is the rate at which electrical energy is used or produced. In plain language, it tells you how much power a device needs to run. A 10 W LED bulb uses far less power than a 1500 W space heater, which is why their energy costs are very different even if they run for the same amount of time.
The unit for power is the watt (W). Larger values are often written in kilowatts (kW), where:
1 kW = 1000 W
Core formulas for calculation of wattage
1) DC circuits
For direct current systems, wattage is straightforward:
- P = V × I
- P = power in watts, V = voltage, I = current in amps
2) Single-phase AC circuits
In alternating current systems, phase difference matters. Use power factor (PF):
- P = V × I × PF
3) Three-phase AC circuits
For balanced three-phase systems:
- P = √3 × V × I × PF
- √3 is approximately 1.732
How to calculate wattage step by step
- Identify circuit type: DC, single-phase AC, or three-phase AC.
- Measure or read voltage and current from meter/nameplate.
- Include power factor for AC equipment (motors, compressors, pumps).
- Apply the correct formula.
- Convert watts to kW if needed by dividing by 1000.
Examples
Example A: DC device
A 24 V DC system draws 3 A.
P = 24 × 3 = 72 W
Example B: Single-phase AC appliance
A 230 V appliance draws 5 A with PF = 0.92.
P = 230 × 5 × 0.92 = 1058 W (about 1.06 kW)
Example C: Three-phase motor
A motor operates at 400 V, draws 12 A, and PF is 0.85.
P = 1.732 × 400 × 12 × 0.85 = 7062 W (about 7.06 kW)
Typical appliance wattage ranges
| Appliance | Typical Wattage | Notes |
|---|---|---|
| LED bulb | 6 - 15 W | Very efficient lighting |
| Laptop charger | 45 - 120 W | Varies with model and load |
| Refrigerator | 100 - 400 W | Cycles on and off |
| Microwave | 800 - 1500 W | High short-term usage |
| Space heater | 1000 - 2000 W | Major contributor to bill |
From wattage to energy cost
Wattage by itself is an instant value. Utility bills are based on energy over time, usually in kilowatt-hours (kWh).
- kWh = (W ÷ 1000) × hours of use
- Cost = kWh × electricity rate
Example: A 1500 W heater running 3 hours/day uses 4.5 kWh/day. If electricity is 0.20 per kWh, daily cost is 0.90.
Common mistakes to avoid
- Ignoring power factor in AC calculations.
- Mixing units (mA vs A, kW vs W).
- Using nameplate current as constant real-world current in all conditions.
- Forgetting startup/inrush current on motors and compressors.
- Calculating energy cost from watts without multiplying by time.
Quick practical guidance
When PF can be assumed as 1.0
Purely resistive loads such as electric heaters and many incandescent lamps are close to PF = 1.0.
When PF must be included
Motors, inductive loads, and many industrial machines typically have PF less than 1, often between 0.7 and 0.95.
Safety note
Calculation is useful for planning, but always follow electrical codes and manufacturer specifications. If you are sizing wiring, breakers, or industrial equipment, consult a licensed electrician or electrical engineer.