  High energy electrons carry much of a solar flare's energy. Therefore,
understanding changes in electron beam distributions during their propagation
is crucial. A key focus of this paper is how the co-spatial return current
reduces the energy flux carried by these accelerated electrons. We
systematically compute this reduction for various beam and plasma parameters
relevant to solar flares. Our 1D model accounts for collisions between beam and
plasma electrons, return current electric-field deceleration, thermalization in
a warm target approximation, and runaway electron contributions. The results
focus on the classical (Spitzer) regime, offering a valuable benchmark for
energy flux reduction and its extent. Return current losses are only negligible
for the lowest nonthermal fluxes. We calculate the conditions for return
current losses to become significant and estimate the extent of the
modification to the beam's energy flux density. We also calculate two
additional conditions which occur for higher injected fluxes: (1) where runaway
electrons become significant, and (2) where current-driven instabilities might
become significant, requiring a model that self-consistently accounts for them.
Condition (2) is relaxed and the energy flux losses are reduced in the presence
of runaway electrons. All results are dependent on beam and co-spatial plasma
parameters. We also examine the importance of the reflection of beam electrons
by the return-current electric field. We show that the interpretation of a
number of flares needs to be reviewed to account for the effects of return
currents.
