Kwh to joules (kWh to J)

One kilowatt-hour is exactly 3,600,000 joules: a thousand watts for three thousand six hundred seconds, and a watt is exactly one joule per second. Nothing here is measured.

Worked through: 2 kWh x 3,600,000 = 7,200,000 J.

Going the other way, one joule is 2.777777778e-7 kilowatt-hours.

For mental arithmetic: Multiply by 3.6 million.

The conversion that makes an electricity bill into physics. A kilowatt-hour is 3.6 megajoules, which is the energy to lift about 367 tonnes by one meter, or to boil roughly ten liters of water from cold. The reason billing uses kWh rather than joules is purely readability: a typical household uses around 10 kWh a day, which is 36 million joules, and nobody wants an eight-digit meter. Note that the kilowatt-hour is a unit of energy despite being built from a unit of power, because multiplying a rate by a time gives an amount.

Precision is worth matching to the input. A measurement given as 0.1 kWh carries one or two significant figures, and reporting its conversion to six implies a precision the original never had.