Out of a photon and an electron, the equation $E = pc$, is valid for

Energy is given by

$E=\frac{m_0{c}^2}{\sqrt{1-\left(v^2/c^2\right)}}\text{ or }{E}^2=\frac{m_0^2{c}^6}{c^2-v^2}$

Momentum p is given by

$\begin{array}{l}p=\frac{m_{0}v}{\sqrt{1-\left(v^2/c^2\right)}}\text{ or }{p}^2{c}^2=\frac{m_0^2{c}^4{v}^2}{c^2-v^2}\\ \therefore E^2-p^2{c}^2={m_0}^2{c}^4\\ \text{ or }{E}^2=p^2{c}^2+{m_0}^2{c}^4\end{array}$

For photon, rest mass

$m_0=0,\text{ so }E=pc$

For electron, $m_0\ne 0,\text{ so }E\ne pc$