# How do you calculate the energy efficiency of a device?

$\eta = \frac{{P}_{o u t}}{{P}_{i n}}$.
But since power is the rate of doing work, or the rate of energy transfer, $P = \frac{W}{t}$, and both the output and input systems operate simultaneously for the same duration of time, we may also give the efficiency as the ratio of the energy output to he energy input.
$\eta = {W}_{o u t} / {W}_{i n}$.
Since in all cases ${W}_{o u t} \le {W}_{i n}$, we ofteh multiply the efficiency $\eta$ by $100$ in order to express it as a percentage