The electrostatic and kinetic energies available in the longitudinal e
lectron plasma wave created in the wake of an ultraintense laser pulse
are obtained analytically. The analysis is one dimensional and assume
s a square-shaped pulse that propagates in a highly underdense plasma
with the velocity of light in vacuum. The length scale for laser deple
tion is given as a function of the laser irradiance and the electron d
ensity.