What Is a Watt?
A watt is a simple idea with a big impact: it tells you how quickly energy is being used or produced. You see watts on light bulbs, phone chargers, speakers, kettles, solar panels, and electric motors. Knowing what watts mean helps you compare devices, estimate electricity costs, and pick the right power supply.
What a Watt Measures
A watt (W) is a unit of power. Power is the rate of energy transfer—how much energy moves or is used per second.
- 1 watt = 1 joule per second (1 W = 1 J/s)
So if a device uses 60 W, it uses energy at a rate of 60 joules every second while running.
Watts vs Energy (Watt-Hours)
Watts measure power, not total energy. For total energy over time, electric bills often use kilowatt-hours (kWh).
- Energy (Wh) = Power (W) × Time (h)
- Energy (kWh) = Power (kW) × Time (h)
Example: A 100 W bulb running for 10 hours:
- 100 W × 10 h = 1000 Wh = 1 kWh
Common Watt Values in Daily Life
Watts give you a quick way to compare how demanding devices are.
- LED bulb: 5–15 W
- Laptop charger: 45–100 W
- Microwave: 800–1500 W
- Electric kettle: 1500–3000 W
- Hair dryer: 1200–2000 W
- Space heater: 1000–2000 W
Higher watts usually mean more power draw (or more power output), but performance also depends on design and efficiency.
How to Calculate Watts
There are several practical ways to calculate watts depending on what information you have.
1) Watts From Voltage and Current (Electrical)
In many basic cases (especially DC), power is:
P = V × I
Where:
- P = power in watts (W)
- V = voltage in volts (V)
- I = current in amperes (A)
Example: A device uses 2 A at 12 V:
- P = 12 × 2 = 24 W
2) AC Power: Add Power Factor
In many AC systems (common for household appliances and motors), real power can be:
P = V × I × PF
Where PF is power factor (a number from 0 to 1). Some devices, especially those with motors or certain power supplies, may have PF below 1.
Example: 230 V, 1.5 A, PF = 0.8:
- P = 230 × 1.5 × 0.8 = 276 W
If PF is not known, a watt meter provides a direct measurement.
3) Watts From Resistance (Ohm’s Law)
If you know resistance (R) and either voltage or current:
- P = V² / R
- P = I² × R
Example: A heater coil with R = 10 Ω on 120 V:
- P = 120² / 10 = 14400 / 10 = 1440 W
4) Mechanical Power (Motors and Motion)
For rotating machines, a common form is:
P = τ × ω
Where:
- τ (tau) = torque (N·m)
- ω (omega) = angular speed (rad/s)
In many practical settings, motor ratings are given directly in watts or horsepower. A rough conversion:
- 1 horsepower ≈ 746 W
How Watts Relate to Electricity Cost
Electric companies bill for energy (kWh), not watts. Still, watts let you estimate cost.
Steps:
- Convert watts to kilowatts: kW = W / 1000
- Multiply by hours used: kWh = kW × h
- Multiply by your rate per kWh.
Example: A 1500 W heater used for 3 hours:
- 1500 W = 1.5 kW
- Energy = 1.5 × 3 = 4.5 kWh
- If electricity costs \$0.20/kWh: 4.5 × 0.20 = \$0.90
Why Watts Matter When Choosing Equipment
Watts help prevent overloads and failures. A power strip, circuit, or power supply must handle the expected wattage (often with extra margin). For batteries and chargers, watt ratings also affect charging speed and runtime.
Watts are simply a measure of power, but they connect directly to performance, safety, and cost—making them one of the most useful numbers on any electrical label.
