Watt
Definition
The watt (symbol W) is the SI unit of power, measuring the rate at which electrical energy is consumed or produced. One watt equals one joule per second, or one ampere of current flowing at one volt. Common power ratings: LED light bulb 8โ15W, laptop 45โ65W, microwave 700โ1200W, hair dryer 1,500โ1,875W, electric oven 2,000โ5,000W. For larger loads, kilowatts (kW = 1,000W) are used. The electricity on your utility bill is measured in kilowatt-hours (kWh) โ the total energy consumed, calculated as watts ร hours รท 1,000.
Why is Watt Important?
In electrical engineering and everyday applications, Watt is a fundamental concept for understanding how electrical systems work. Whether you are an engineer designing circuits, an electrician sizing wires, or a homeowner estimating energy costs, this metric is essential for safety, efficiency, and accurate calculations.
Our electrical conversion calculators help you quickly convert between units and verify calculations, reducing errors and saving time in both professional and DIY electrical work.
What is a Watt?
The watt (symbol W) is the SI unit of power โ the rate at which electrical energy is used or produced. One watt equals one joule of energy per second, or one ampere of current flowing at one volt. Watts tell you how much energy a device uses right now, while kilowatt-hours tell you how much total energy it uses over time.
Common Device Wattages
| Device | Watts | Amps at 120V | Monthly Cost* |
|---|---|---|---|
| LED light bulb | 8โ15 W | 0.07โ0.13 A | $0.30โ$0.55 |
| Ceiling fan | 10โ75 W | 0.08โ0.63 A | $0.35โ$2.70 |
| Laptop | 45โ65 W | 0.38โ0.54 A | $1.60โ$2.35 |
| Desktop computer | 150โ500 W | 1.25โ4.2 A | $5.40โ$18 |
| TV (55" LED) | 60โ120 W | 0.5โ1.0 A | $2.15โ$4.30 |
| Refrigerator | 100โ400 W | 0.8โ3.3 A | $3.60โ$14.40 |
| Microwave | 700โ1,200 W | 5.8โ10 A | $2.50โ$4.30 |
| Hair dryer | 1,500โ1,875 W | 12.5โ15.6 A | $1.60โ$2.00 |
| Space heater | 1,500 W | 12.5 A | $54 |
| Electric oven | 2,000โ5,000 W | 8.3โ20.8 A (240V) | $14โ$36 |
| Central AC (3 ton) | 3,000โ3,500 W | 12.5โ14.6 A (240V) | $72โ$84 |
*Monthly cost based on 8 hrs/day use at $0.16/kWh US average (2024), except fridge (24/7), space heater (8h), microwave (30min/day)
Key Power Formulas
| Find | Formula |
|---|---|
| Watts | W = V ร A (Volts ร Amps) |
| Watts from Resistance | W = Vยฒ รท R or W = Iยฒ ร R |
| kWh from Watts | kWh = W ร hours รท 1,000 |
| Cost | Cost = kWh ร rate per kWh |