Watt and kilowatt

The idea

The SI unit of electric power is the watt (W), the power consumed by a device carrying 1 A at a potential difference of 1 V, so 1 W = 1 V A; a kilowatt equals 1000 watts.

Reading a label

A bulb is marked 110 W. A large machine is marked 2 kW.

How many watts is 1 kW?

• 1000 W
• 100 W
• 10 W

Match them up

Tap a description on the left, then the value on the right that goes with it.

A small unit and a big one

The watt is a small unit, so for bigger machines we use the kilowatt. A watt is also a volt multiplied by an ampere, because P = VI.

The SI unit of electric power is the watt (W), the power consumed by a device carrying 1 A at a potential difference of 1 V, so 1 W = 1 V A. A kilowatt equals 1000 watts.

Notes

The SI unit of power is the watt (W): 1 W = 1 V A, the power of a device carrying 1 A at 1 V. A kilowatt is 1000 watts.

Check yourself

An electric bulb is connected to a 220 V source and a current of 0.50 A flows. What is the power of the bulb, in W?

Answer: 110 W

P = VI = 220 V × 0.50 A = 110 W.

A heater is rated 2.5 kW. What is its power in W?

Answer: 2500 W

1 kW = 1000 W, so 2.5 kW = 2.5 × 1000 W = 2500 W.

Which statement about the watt is correct?

Why is the kilowatt used in actual practice?

• 1 W is 1 volt multiplied by 1 ampere — correct. Yes!
• 1 W is 1 volt divided by 1 ampere. That would be 1 ohm. The watt is the product V × A.
• 1 W is 1 coulomb in 1 second. That is 1 ampere, the unit of current. The watt is the unit of power, 1 V A.
• Because 1 kW is smaller than 1 W. A kilowatt is 1000 watts, much larger than a watt.
• The watt is a very small unit, and 1 kW is equal to 1000 W — correct. Yes!
• Because the watt is not an SI unit. The watt is the SI unit of electric power. The kilowatt is used only because the watt is very small.