Watt Calculator (Power, Voltage, Current)
Enter any two values (W, V, A) to calculate the missing one. Optionally add usage hours and electricity rate to estimate energy cost.
If you have ever looked at an appliance label and wondered what "W" actually means, this guide is for you. A proper calcul watt helps you understand electrical power, compare devices, estimate energy use, and avoid costly surprises on your bill.
What is a watt?
A watt (W) is a unit of power. It tells you how fast electrical energy is being used at a moment in time. Higher wattage generally means a device is consuming energy faster.
- Power (W): instant electrical demand
- Energy (kWh): power used over time
- Cost: energy multiplied by your utility rate
People often confuse watts and kilowatt-hours. Think of watts like speed (km/h or mph), and kWh like total distance traveled.
Core formulas for calcul watt
For basic DC or simplified AC calculations
- W = V × A
- V = W ÷ A
- A = W ÷ V
These three formulas are the backbone of quick electrical calculations for many household scenarios.
For AC appliances with power factor
Some devices (motors, compressors, power supplies) do not convert all apparent power to real power. In those cases:
- Real Power (W) = V × A × PF
Where PF is power factor (typically from 0 to 1). If you do not know PF, your estimate may be less accurate for inductive loads.
How to use this calculator correctly
- Enter any two of these fields: Power (W), Voltage (V), Current (A).
- Click Calculate to get the missing value.
- Add hours/day to estimate daily and monthly energy consumption.
- Add cost per kWh to estimate daily and monthly operating cost.
Tip: if you already know watts from a product label, enter only watts plus hours/day and rate to estimate running costs.
Practical examples
Example 1: LED lamp
A 10W LED running 6 hours/day uses:
- Daily energy: 10 × 6 / 1000 = 0.06 kWh
- Monthly energy (~30 days): 1.8 kWh
Example 2: Electric heater
A 1500W heater used 4 hours/day:
- Daily energy: 1500 × 4 / 1000 = 6 kWh
- Monthly energy: 180 kWh
At $0.20/kWh, that is about $36/month for this one device.
Example 3: Finding amperage
If a 1200W appliance runs on 230V:
- A = 1200 ÷ 230 = 5.22A (approx.)
This is useful for breaker sizing checks and extension cord planning.
Watts to kWh to cost: the full chain
To convert device power into money, use the sequence below:
- kWh/day = (W × hours/day) ÷ 1000
- Cost/day = kWh/day × rate
- Cost/month ≈ Cost/day × 30
This lets you quickly compare "always-on" electronics vs. intermittent high-power devices.
Common mistakes in watt calculations
- Mixing up watts and watt-hours
- Ignoring duty cycle (device not running continuously)
- Using nominal power instead of measured power
- Forgetting standby consumption
- Ignoring power factor for certain AC loads
If you want high precision, use a plug-in energy meter and compare real measured usage against label values.
Typical appliance wattage ranges
- LED bulb: 5–12W
- Laptop charger: 45–100W
- Desktop computer: 150–500W
- Microwave: 800–1500W
- Hair dryer: 1000–2000W
- Electric kettle: 1500–3000W
- Space heater: 1000–2000W
These are broad estimates; actual consumption depends on model, settings, and use pattern.
Safety note
A calcul watt helps planning, but electrical safety comes first. Do not overload outlets, extension leads, or breakers. If you are unsure about wiring, panel limits, or sustained current loads, consult a qualified electrician.
Final takeaway
Understanding watts is one of the simplest and most powerful ways to control energy use. With a quick calculation, you can size equipment, estimate bills, and make better purchasing decisions. Use the calculator above whenever you need a fast and reliable watt estimate.