Watt (W) – The Unit of Electrical Power
What is Watt?
Watt (symbol: W) is the SI unit for power. It describes the energy conversion per time period and is named after the Scottish inventor James Watt. In electrical engineering, watt indicates how much electrical energy per second is converted into other forms of energy (light, heat, motion).
The unit watt is derived from the SI base units kilogram (kg), meter (m), and second (s):
1 W = 1 kg · m² / s³
Or more simply put: 1 watt = 1 joule per second (1 W = 1 J/s)
For electrical devices, the power in watts equals the product of voltage (volts) and current (amperes):
P = U × I (Power = Voltage × Current)
Watt in Lighting Technology
In traditional lighting with incandescent bulbs, wattage was the most important indicator of brightness. A 60 W bulb was brighter than a 40 W bulb – power consumption directly correlated with light output.
With modern LED light sources, this has fundamentally changed. LEDs are much more energy-efficient and produce the same brightness at significantly lower wattage. That's why lumens (lm) and lux (lx) are now the decisive metrics for light output, while watts only describe power consumption.
Watt vs. Lumen – The LED Comparison
| Old Incandescent Bulb (Watt) | LED Power (Watt) | Luminous Flux (Lumen) | Energy Savings |
|---|---|---|---|
| 25 W | 3–4 W | approx. 250 lm | approx. 85% |
| 40 W | 5–6 W | approx. 470 lm | approx. 87% |
| 60 W | 8–10 W | approx. 800 lm | approx. 85% |
| 75 W | 10–13 W | approx. 1,050 lm | approx. 85% |
| 100 W | 13–17 W | approx. 1,500 lm | approx. 85% |
What is Measured in Watts?
The unit watt is used for various types of power. Here's an overview of the most important applications in electrical engineering:
| Application Area | Typical Power (Watt) | Examples |
|---|---|---|
| LED Light Sources | 3–20 W | E27 LED, GU10 spots, LED panels |
| Halogen Lamps | 20–100 W | Halogen spotlights, low-voltage halogen |
| Electric Motors | 100–10,000 W | Industrial motors, pumps, compressors |
| Heating Devices | 1,000–3,000 W | Fan heaters, infrared heaters, radiators |
| Household Appliances | 500–2,500 W | Kettles, vacuum cleaners, microwaves |
| IT Equipment | 50–500 W | Computers, servers, monitors |
| Photovoltaic Modules | 300–450 W | Solar panels (rated power per panel) |
Watt Units: Conversion and Table
In practice, different multiples of the watt unit are used depending on the power range. Here are the most important conversions:
| Unit | Abbreviation | Conversion | Application Area |
|---|---|---|---|
| Milliwatt | mW | 1 mW = 0.001 W | Laser technology, small sensors |
| Watt | W | 1 W (base unit) | Light sources, small devices |
| Kilowatt | kW | 1 kW = 1,000 W | Electric motors, heaters, house connections |
| Megawatt | MW | 1 MW = 1,000 kW = 1,000,000 W | Industrial plants, wind turbines |
| Gigawatt | GW | 1 GW = 1,000 MW = 1,000,000,000 W | Power plants, large energy grids |
Active Power, Apparent Power, and Reactive Power
In AC technology, distinguishing between different types of power is important:
- Active Power (P) – measured in watts (W): The power actually converted into work (e.g., light, heat, mechanical work)
- Apparent Power (S) – measured in volt-amperes (VA): The total power provided by the grid
- Reactive Power (Q) – measured in var (volt-ampere reactive): The power that is not usable but circulates in the grid
Active power in watts is the only quantity that is actually converted into usable energy. With purely resistive loads (e.g., incandescent bulbs, heaters), active power equals apparent power. With inductive or capacitive loads (e.g., motors, ballasts), the values differ.
Frequently Asked Questions (FAQ)
What does watt mean for electrical devices?
Watt indicates the electrical power that a device consumes. The higher the wattage, the more energy is converted per second. A 2,000 W kettle converts 2,000 joules of energy per second into heat.
How do you calculate watts?
You calculate power in watts using the formula: P = U × I (Power = Voltage × Current). Example: A device at 230 V drawing 2 A has a power of 230 V × 2 A = 460 W.
What is the difference between watt and kilowatt?
Kilowatt (kW) is a multiple of watt. 1 kW = 1,000 W. Kilowatt is used for larger powers, such as electric motors, heaters, or house connections. A 5 kW motor has a power of 5,000 W.
Why is wattage less important for LEDs?
For LEDs, wattage only indicates power consumption, not brightness. A 10 W LED can be just as bright as an old 60 W incandescent bulb. The crucial factor is the lumen value (lm), which indicates actual light output.
What is the difference between watts and watt-hours?
Watt (W) is power – energy per time. Watt-hours (Wh) or kilowatt-hours (kWh) describe the amount of energy over a specific time period. Example: A 100 W lamp consumes 1,000 Wh = 1 kWh in 10 hours.
What role does wattage play in sizing cables?
Wattage helps calculate the required cable cross-section. Using P = U × I, you can determine the current and derive the necessary cross-section according to electrical codes. At 230 V and 3,000 W, approximately 13 A flows – a 1.5 mm² cable would suffice for short distances.
Summary
Watt (W) is the SI unit for power and describes energy conversion per time period. In electrical engineering, watt indicates active power – the actually usable electrical power. The unit is derived from the base units kilogram, meter, and second: 1 W = 1 kg · m² / s³.
For modern LED light sources, wattage is less meaningful than before. Brightness is now indicated in lumens (lm), while watts only describe power consumption. For practical electrical installation work, wattage remains important – for calculating currents, cable cross-sections, and energy consumption.