We show that gamma-ray burst (GRB) afterglow observations strongly suggest, within the fireball model framework, that radiating electrons are shock accelerated to a power-law energy distribution, with universal index p \approx 2.2, and that the fraction of shock energy carried by electrons, ��_e, is universal and close to equipartition, ��_e ~ 1/3. For universal p and ��_e, a single measurement of the X-ray afterglow flux on the time scale of a day provides a robust estimate of the fireball energy per unit solid angle, ��, averaged over a conical section of the fireball of opening angle ��~ 0.1. Applying our analysis to BeppoSAX afterglow data we find that: (i) Fireball energies are in the range of 4����=10^{51.5} to 10^{53.5} erg; (ii) The ratio of observed $��$-ray to total fireball energy per unit solid angle, ��_��/ ��, is of order unity, satisfying abs[log10(��_��/��)]<0.5; (iii) If fireballs are jet like, their opening angle should satisfy ��>=0.1. Our results imply that if typical opening angles are ��~ 0.1, a value consistent with our analysis, the total energy associated with a GRB event is in the range of 10^{50} erg to 10^{51.5} erg.

16 pages; Submitted to ApJ