Calculating Energy Costs

Cost is based on energy consumed; thus, we must find $E$ from $E=\text{Pt}$ and then calculate the cost. Because electrical energy is expressed in $\text{kW}\cdot \mathrm{h}$, at the start of a problem such as this it is convenient to convert the units into $\text{kW}$ and hours.

The energy consumed in $\text{kW}\cdot \text{h}$ is

and the cost is simply given by

The cost of using the computer in this example is neither exorbitant nor negligible. It is clear that the cost is a combination of power and time. When both are high, such as for an air conditioner in the summer, the cost is high.