Problem — energy of a laptop computer

Problem

Estimate the electrical energy consumed by a $50\ \text{W}$ laptop over one hour of use. How much does it cost, knowing that $1\ \text{kWh}$ costs $\sim 0{,}22$ euros?

Solution

Energy consumed. Energy is power times time. With $P = 50\ \text{W}$ and $t = 1\ \text{h} = 3600\ \text{s}$: $E = P\,t = 50\ \text{W} \cdot 3600\ \text{s} = 1{,}8\cdot10^{5}\ \text{J} \approx 200\ \text{kJ}$

In kilowatt-hours: it is convenient to use $E = P\,t$ directly with the power in kW and the time in hours: $E = 0{,}050\ \text{kW} \cdot 1\ \text{h} = 0{,}050\ \text{kWh}$

Cost: $\text{cost} = 0{,}050\ \text{kWh} \cdot 0{,}22\ \text{euro/kWh} \approx 0{,}011\ \text{euro}$ just over a cent: an hour of laptop use costs a negligible fraction.

$\ev{E \approx 1{,}8\cdot10^{5}\ \text{J} \sim 200\ \text{kJ} \qquad \text{cost} \approx 0{,}011\ \text{euro}}$

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