What is the actual formula for electric power?

If a current, I flows through a conductor for a time, t against a potential difference, V then, work done by the current,

$W=V I t$

Thus, electric power, $P=\frac{W}{t}=\frac{V I t}{t}=V I or, P= V I$  ...........(i)

Further as, $V=I R$

Thus,  $P=(I R) I=I^{2}R or, P= I^{2}R$     ............(ii)

And as, $I=\frac{V}{R}$

Thus,  $P=V\left(\frac{V}{R}\right)=V^{2}R or, P=V^{2}R$   ...........(iii)

All the three equations are used as the formula for calculating electric power. However, equation (ii) is used for calculating electric power in case current (I) is constant and equation (iii) is ​used for calculating electric power if potential difference (V) is constant. Equation (i) can be used in all cases where either V or I changes or both change.