Calculation:  lightning's radiant heat energy relative to the Sun

We calculate lightning's heat energy relative to the Sun's as follows:

The energy radiated by a hot object per second is proportional to the fourth power of the absolute temperature and is expressed by this equation:

P = e σ A T⁴

where e is the emissivity of the object (e = 1 for ideal radiator), σ = 5.6703 x 10^-8 watt/m² and A is the surface area of the radiating body.

Thus, for each second, the ratio of the energy radiated by the Sun (P_s) to that radiated by lightning is (P_l) is given by

P_s / P_l = 10^-4 A_s / A_s

assuming that the lightning temperature is 10 times the Sun's temperature and that both are ideal radiators.

The Sun's surface area is 0.609 x 10¹⁹ m².

The surface area of a typical lightning bolt is 314 m², assuming a bolt length of 100 m and a diameter of 2 m. 

Thus P_s / P_l = 7.66 x 10¹⁶ ~ 10¹⁷

Only one billionth of the Sun's radiant energy reaches Earth.  So the Sun's radiant energy that reaches Earth is 10⁸  times a lightning's bolt radiant energy per unit second.