Daily Physicist

Today is the birthday of

• Joseph von Fraunhofer (1787-1826).

Today’s Problem

Kilowatt-hours (kWh) is a unit of energy used in quantifying electrical energy usage as supplied by electric utilities. How many joules is equivalent to a kilowatt-hour? The average American household consumes 30 kWh of energy per day. What is this in joules?

Answer

\[1\,\text{kW}\cdot\text{hr}=1000\,\text{W}\cdot\text{hr}=1000\,\text{J/s}\cdot\text{hr}\left(\frac{3600\,\text{s}}{\text{hr}}\right)=3.6\times 10^6\,\text{J}\]

\[30\,\text{kW}\cdot\text{hr}\left(\frac{3.6\times 10^6\,\text{J}}{1\,\text{kW}\cdot\text{hr}}\right)=108\times 10^6\,\text{J}\]

So a kilowatt-hour is equivalent to 3.6 million joules, and the average daily American household energy consumption is 108 million joules.