400w

400 watts (400 W) is a unit of power used to express the rate of energy transfer or conversion. In electrical contexts, it denotes the amount of electrical power a device draws from the supply or the amount of mechanical or light energy it can deliver per second. The watt is defined as one joule per second, so 400 W equals 400 joules of energy transferred each second.

In practice, a device labeled 400 W is assumed to operate near that rate under specified conditions. The actual consumption depends on voltage and current and, for AC systems, the power factor. Real power is P = V × I × cos(φ). At standard mains voltages, a 400 W device would draw roughly 3.3 A at 120 V (if cos φ ≈ 1) or about 1.7 A at 230 V.

Energy use over time is 0.4 kilowatt-hours per hour of operation. A 400 W device running for two hours consumes 0.8 kWh, which factors into energy costs and cooling requirements.

Applications range from consumer electronics and tools to lighting and small heaters. For example, a compact power supply or a desktop computer component may be rated around 400 W, while LED or HID lighting in the hundreds of watts produces significant lumen output with varying efficiency. Efficiency and design determine whether a device delivers its nominal 400 W output consistently.

Related units include the kilowatt (1,000 W) and mechanical horsepower (about 745.7 W).