According to the Stefan-Boltzmann law, the power radiated per unit area ($E$) by a black body is directly proportional to the fourth power of its absolute temperature ($T$). The relation is given by $E = \sigma T^4$, where $\sigma$ is the Stefan-Boltzmann constant.

1. Rearranging the formula: $\sigma = E / T^4$
2. Substituting Units: Power per unit area ($E$): Watt per square metre ($W m^{-2}$) Temperature ($T$): Kelvin ($K$)
3. Determining the Unit of $\sigma$: $\text{Unit} = \frac{W m^{-2}}{K^4} = W m^{-2} K^{-4}$ This value is explicitly listed in the NCERT Appendices as $5.670 \times 10^{-8} W m^{-2} K^{-4}$ .