Calculating Input power from a source
The efficiency is:

$$\eta = \dfrac{p_{out}}{p_{in}} = \dfrac{V_{out}\cdot I_{out}}{V_{in}\cdot I_{in}}$$

For a linear regulator, assuming the output current is much greater than the regulator circuit's internal currents, we have \$I_{out} \approx I_{in}\$ thus:

$$\eta \approx  \dfrac{V_{out}}{V_{in}}$$