The energy of a Landau-damped electrostatic wave is a long-standing
problem. Calculations based on a spatially infinite wave are deficient. In this
paper, a wave packet is analysed. The energy density depends on second-order
initial conditions that are independent of the first-order wave. Pictures of
energy and momentum transfer to resonant electrons are presented. On
physical grounds, suitable second-order initial conditions are proposed. The
resulting wave energy agrees with fluid theory. The ratio between energy and
momentum is not the phase velocity up, as predicted by
fluid theory, but ½up,
in agreement with Landau-damping physics.