Kilowatt hour

The idea

Electrical energy is power × time, measured in watt hours; its commercial unit is the kilowatt hour (kW h), commonly called a 'unit', where 1 kW h = 3.6 × 10⁶ J.

What is a unit?

On an electricity bill the energy used is counted in 'units'. A device of 1000 W runs for 1 hour.

How many 'units' of energy does it use?

• 1 unit, which is 1 kW h
• 1000 units
• 3600 units

Power times time

Pick the power and the time. The energy is power multiplied by time.

The unit on the bill

Energy is power times time. If the power is in watts and the time in hours, the energy comes in watt hours. Its commercial unit is the kilowatt hour, commonly called a unit.

Electrical energy is power × time, measured in watt hours. Its commercial unit is the kilowatt hour (kW h), commonly called a unit, where 1 kW h = 3.6 × 10⁶ J.

Notes

Electrical energy is power × time, in watt hours; the commercial unit is the kilowatt hour (kW h), called a unit, with 1 kW h = 3.6 × 10⁶ J.

Check yourself

An electric refrigerator rated 400 W runs for 8 hours each day. How much energy does it use in 30 days, in kW h?

Answer: 96 kW h

400 W × 8 h × 30 days = 96000 W h = 96 kW h.

The refrigerator in the last question used 96 kW h. The cost of energy is Rs 3.00 for each kW h. What is the cost, in Rs?

Answer: 288 Rs

Cost = 96 kW h × Rs 3.00 per kW h = Rs 288.

What is 1 kW h equal to?

What is the 'unit' used in electricity bills?

• 3.6 × 10³ J. 1 kW h = 1000 W × 3600 s = 3.6 × 10⁶ J, which is a thousand times more than this.
• 1000 J. 1 kW h is 1000 watts for 3600 seconds, which is 3.6 × 10⁶ J.
• 3.6 × 10⁶ J — correct. Yes!
• The kilowatt hour — correct. Yes!
• The watt. The watt is the unit of power. The commercial unit of electric energy, called a unit, is the kilowatt hour.
• The ampere. The ampere is the unit of current. The unit of energy on a bill is the kilowatt hour.