  Solar flare hard X-ray spectroscopy serves as a key diagnostic of the
accelerated electron spectrum. However, the standard approach using the
collisional cold thick-target model poorly constrains the lower-energy part of
the accelerated electron spectrum, and hence the overall energetics of the
accelerated electrons are typically constrained only to within one or two
orders of magnitude. Here we develop and apply a physically self-consistent
warm-target approach which involves the use of both hard X-ray spectroscopy and
imaging data. The approach allows an accurate determination of the electron
distribution low-energy cutoff, and hence the electron acceleration rate and
the contribution of accelerated electrons to the total energy released, by
constraining the coronal plasma parameters. Using a solar flare observed in
X-rays by the {\em RHESSI} spacecraft, we demonstrate that using the standard
cold-target methodology, the low-energy cutoff (and hence the energy content in
electrons) is essentially undetermined. However, the warm-target methodology
can determine the low-energy electron cutoff with $\sim$7\% uncertainty at the
$3\sigma$ level and hence permits an accurate quantitative study of the
importance of accelerated electrons in solar flare energetics.
