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 8–15W, laptop 45–65W, microwave 700–1200W, hair dryer 1,500–1,875W, electric oven 2,000–5,000W. For larger loads, kilowatts (kW = 1,000W) are used. The electricity on your utility bill is measured in kilowatt-hours (kWh) — the total energy consumed, calculated as watts × hours ÷ 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 — 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 | 8–15 W | 0.07–0.13 A | $0.30–$0.55 |
| Ceiling fan | 10–75 W | 0.08–0.63 A | $0.35–$2.70 |
| Laptop | 45–65 W | 0.38–0.54 A | $1.60–$2.35 |
| Desktop computer | 150–500 W | 1.25–4.2 A | $5.40–$18 |
| TV (55" LED) | 60–120 W | 0.5–1.0 A | $2.15–$4.30 |
| Refrigerator | 100–400 W | 0.8–3.3 A | $3.60–$14.40 |
| Microwave | 700–1,200 W | 5.8–10 A | $2.50–$4.30 |
| Hair dryer | 1,500–1,875 W | 12.5–15.6 A | $1.60–$2.00 |
| Space heater | 1,500 W | 12.5 A | $54 |
| Electric oven | 2,000–5,000 W | 8.3–20.8 A (240V) | $14–$36 |
| Central AC (3 ton) | 3,000–3,500 W | 12.5–14.6 A (240V) | $72–$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 × A (Volts × Amps) |
| Watts from Resistance | W = V² ÷ R or W = I² × R |
| kWh from Watts | kWh = W × hours ÷ 1,000 |
| Cost | Cost = kWh × rate per kWh |