Table of Contents
What Is a Watt?
The watt (W) is the SI unit of power, defined as one joule per second. It quantifies the rate at which energy is transferred or converted. Named after Scottish inventor James Watt, the unit is used universally in electrical engineering, mechanical systems, and thermodynamics to express the rate of energy consumption or production.
In electrical circuits, power in watts equals the product of voltage (in volts) and current (in amperes). For AC circuits with reactive components, the power factor must be included to get the real (active) power consumed by the load.
Power Formulas
These three forms of the power equation allow you to calculate power from any combination of voltage, current, and resistance. The power factor (PF) accounts for the phase difference between voltage and current in AC circuits, ranging from 0 to 1.
Common Wattage Reference
| Device | Typical Watts | Daily kWh (8h) |
|---|---|---|
| LED bulb | 10 W | 0.08 |
| Laptop | 50 W | 0.40 |
| Refrigerator | 150 W | 1.20 |
| Microwave | 1000 W | 8.00 |
| Electric heater | 1500 W | 12.00 |
| Air conditioner | 3500 W | 28.00 |
Watt Conversions
- 1 kilowatt (kW) = 1,000 watts
- 1 megawatt (MW) = 1,000,000 watts
- 1 horsepower (hp) = 745.7 watts
- 1 watt = 3.412 BTU/hr
- 1 watt = 1 joule/second
Frequently Asked Questions
What is the difference between watts and watt-hours?
Watts measure power (rate of energy use), while watt-hours measure energy (total amount used). A 100W bulb running for 10 hours uses 1,000 Wh (1 kWh). Your electricity bill is based on kilowatt-hours, not watts.
What is power factor?
Power factor is the ratio of real power (watts) to apparent power (volt-amps) in an AC circuit. A power factor of 1 means all power is used as real power. Inductive loads like motors typically have power factors of 0.7-0.9, meaning some power oscillates without doing useful work.
How many watts does a typical home use?
The average US home uses about 1.2 kW on average (10,500 kWh per year). Peak demand can reach 5-10 kW when air conditioning, cooking, and laundry appliances run simultaneously.